processor: introduce PostBuilder registry to replace hardcoded switch

Replace the large, duplicate switch statements in planPost and commitPlan
with a PostBuilder interface registered per file extension.

- PostBuilder interface: Plan(srcPath, ext) (postPlan, error) and
  Commit(plan, postDir, id, now) (*post.Post, []string, error)
- Per-type builders: txtBuilder, mdBuilder, imageBuilder, audioBuilder.
- Each builder self-registers via init() into a map[string]PostBuilder.
- Core processor loops are now extension-agnostic, satisfying OCP/DIP.
- All existing tests pass.

5 files changed, 189 insertions, 105 deletions

diff --git a/internal/processor/audio.go b/internal/processor/audio.go
index 68938cf..821b747 100644
--- a/internal/processor/audio.go
+++ b/internal/processor/audio.go
@@ -4,8 +4,47 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
+	"time"
+
+	"codeberg.org/snonux/snonux/internal/post"
 )
 
+type audioBuilder struct{}
+
+func (audioBuilder) Plan(srcPath string, ext string) (postPlan, error) {
+	plan := postPlan{srcPath: srcPath, ext: ext}
+	if err := validateAudio(srcPath); err != nil {
+		return postPlan{}, err
+	}
+	return plan, nil
+}
+
+func (audioBuilder) Commit(plan postPlan, postDir string, id string, now time.Time) (*post.Post, []string, error) {
+	outName := filepath.Base(plan.srcPath)
+	dst := filepath.Join(postDir, outName)
+	if err := copyFile(plan.srcPath, dst); err != nil {
+		return nil, nil, err
+	}
+	src := fmt.Sprintf("posts/%s/%s", id, outName)
+	html := fmt.Sprintf(
+		`<audio controls class="post-audio"><source src="%s" type="audio/mpeg">Your browser does not support audio.</audio>`,
+		src,
+	)
+	p := &post.Post{
+		ID:        id,
+		Timestamp: now,
+		PostType:  post.TypeAudio,
+		Content:   html,
+		Assets:    []string{outName},
+	}
+	return p, nil, nil
+}
+
+func init() {
+	register(".mp3", audioBuilder{})
+}
+
 // validateAudio confirms the audio source file exists and is readable.
 func validateAudio(srcPath string) error {
 	f, err := os.Open(srcPath)
diff --git a/internal/processor/image.go b/internal/processor/image.go
index a981e85..f68f533 100644
--- a/internal/processor/image.go
+++ b/internal/processor/image.go
@@ -8,7 +8,9 @@ import (
 	"image/png"
 	"os"
 	"path/filepath"
+	"time"
 
+	"codeberg.org/snonux/snonux/internal/post"
 	"golang.org/x/image/draw"
 )
 
@@ -17,6 +19,41 @@ const (
 	jpegQuality   = 80
 )
 
+type imageBuilder struct{}
+
+func (imageBuilder) Plan(srcPath string, ext string) (postPlan, error) {
+	plan := postPlan{srcPath: srcPath, ext: ext}
+	img, err := validateImage(srcPath)
+	if err != nil {
+		return postPlan{}, err
+	}
+	plan.validatedImage = img
+	return plan, nil
+}
+
+func (imageBuilder) Commit(plan postPlan, postDir string, id string, now time.Time) (*post.Post, []string, error) {
+	if err := writeImageAsset(plan.validatedImage, postDir); err != nil {
+		return nil, nil, err
+	}
+	src := fmt.Sprintf("posts/%s/image.jpg", id)
+	html := fmt.Sprintf(`<img src="%s" alt="" class="post-image">`, src)
+	p := &post.Post{
+		ID:        id,
+		Timestamp: now,
+		PostType:  post.TypeImage,
+		Content:   html,
+		Assets:    []string{"image.jpg"},
+	}
+	return p, nil, nil
+}
+
+func init() {
+	register(".png", imageBuilder{})
+	register(".jpg", imageBuilder{})
+	register(".jpeg", imageBuilder{})
+	register(".gif", imageBuilder{})
+}
+
 // validateImage reads and decodes the source image, resizing if necessary.
 // It performs only read validation; the caller is responsible for writing assets.
 func validateImage(srcPath string) (image.Image, error) {
diff --git a/internal/processor/markdown.go b/internal/processor/markdown.go
index c258e4c..ee38bf7 100644
--- a/internal/processor/markdown.go
+++ b/internal/processor/markdown.go
@@ -7,12 +7,60 @@ import (
 	"path/filepath"
 	"regexp"
 	"strings"
+	"time"
 
+	"codeberg.org/snonux/snonux/internal/post"
 	"github.com/yuin/goldmark"
 	"github.com/yuin/goldmark/extension"
 	"github.com/yuin/goldmark/renderer/html"
 )
 
+type mdBuilder struct{}
+
+func (mdBuilder) Plan(srcPath string, ext string) (postPlan, error) {
+	plan := postPlan{srcPath: srcPath, ext: ext}
+	html, locals, err := processMd(srcPath)
+	if err != nil {
+		return postPlan{}, err
+	}
+	plan.mdHTML = html
+	plan.localImages = locals
+	return plan, nil
+}
+
+func (mdBuilder) Commit(plan postPlan, postDir string, id string, now time.Time) (*post.Post, []string, error) {
+	html := plan.mdHTML
+	for _, name := range plan.localImages {
+		html = strings.ReplaceAll(html,
+			fmt.Sprintf(`src="%s"`, name),
+			fmt.Sprintf(`src="posts/%s/%s"`, id, name))
+	}
+
+	var inboxExtras []string
+	sourceDir := filepath.Dir(plan.srcPath)
+	for _, name := range plan.localImages {
+		src := filepath.Join(sourceDir, name)
+		dst := filepath.Join(postDir, name)
+		if err := copyFile(src, dst); err != nil {
+			return nil, nil, fmt.Errorf("copy markdown asset %s: %w", name, err)
+		}
+		inboxExtras = append(inboxExtras, src)
+	}
+
+	p := &post.Post{
+		ID:        id,
+		Timestamp: now,
+		PostType:  post.TypeMarkdown,
+		Content:   html,
+		Assets:    plan.localImages,
+	}
+	return p, inboxExtras, nil
+}
+
+func init() {
+	register(".md", mdBuilder{})
+}
+
 // isSimpleImageRef returns true for a filename-only reference (e.g.
 // "img.png") that is safe to treat as a flat local file in the same
 // directory as the markdown source. It rejects subdirectories, absolute
diff --git a/internal/processor/processor.go b/internal/processor/processor.go
index 0c03d86..bb3a84d 100644
--- a/internal/processor/processor.go
+++ b/internal/processor/processor.go
@@ -32,6 +32,31 @@ import (
 	"codeberg.org/snonux/snonux/internal/post"
 )
 
+// PostBuilder is the abstraction used to validate and commit a single post type.
+// Each concrete builder handles one file extension (e.g. .txt, .png, .mp3).
+// Registering a new builder is enough to add support for a new type — no changes
+// to the core planning or commit loops are required.
+type PostBuilder interface {
+	// Plan validates the source file and returns everything needed to commit it later.
+	 Plan(srcPath string, ext string) (postPlan, error)
+	// Commit performs the mutations for this post type and returns the populated Post,
+	// plus any extra inbox files that should be cleaned up after a successful save.
+	Commit(plan postPlan, postDir string, id string, now time.Time) (*post.Post, []string, error)
+}
+
+// builders maps a lower-case file extension to its PostBuilder.
+var builders = make(map[string]PostBuilder)
+
+// register adds a PostBuilder for the given extension. Panics on duplicates so
+// misconfiguration is caught at start-up.
+func register(ext string, b PostBuilder) {
+	ext = strings.ToLower(ext)
+	if _, exists := builders[ext]; exists {
+		panic(fmt.Sprintf("duplicate PostBuilder for extension %q", ext))
+	}
+	builders[ext] = b
+}
+
 // Run scans cfg.InputDir and processes every eligible file into a post directory
 // under cfg.OutputDir/posts/. It uses a two-phase commit pattern:
 //
@@ -97,46 +122,22 @@ type postPlan struct {
 	mdHTML         string
 	localImages    []string
 	validatedImage image.Image
+	builder        PostBuilder
 }
 
 // planPost validates a single source file and returns a plan containing
 // everything needed to commit it later. It performs no mutations.
 func planPost(srcPath string) (postPlan, error) {
 	ext := strings.ToLower(filepath.Ext(srcPath))
-	plan := postPlan{srcPath: srcPath, ext: ext}
-
-	switch ext {
-	case ".txt":
-		html, err := processTxt(srcPath)
-		if err != nil {
-			return postPlan{}, err
-		}
-		plan.textHTML = html
-
-	case ".md":
-		html, locals, err := processMd(srcPath)
-		if err != nil {
-			return postPlan{}, err
-		}
-		plan.mdHTML = html
-		plan.localImages = locals
-
-	case ".png", ".jpg", ".jpeg", ".gif":
-		img, err := validateImage(srcPath)
-		if err != nil {
-			return postPlan{}, err
-		}
-		plan.validatedImage = img
-
-	case ".mp3":
-		if err := validateAudio(srcPath); err != nil {
-			return postPlan{}, err
-		}
-
-	default:
+	b, ok := builders[ext]
+	if !ok {
 		return postPlan{}, fmt.Errorf("unsupported file type: %s", ext)
 	}
-
+	plan, err := b.Plan(srcPath, ext)
+	if err != nil {
+		return postPlan{}, err
+	}
+	plan.builder = b
 	return plan, nil
 }
 
@@ -153,79 +154,10 @@ func commitPlan(plan postPlan, postsDir string, now time.Time) error {
 		return fmt.Errorf("create post dir %s: %w", id, err)
 	}
 
-	var p *post.Post
-	var inboxExtras []string
-
-	switch plan.ext {
-	case ".txt":
-		p = &post.Post{
-			ID:        id,
-			Timestamp: now,
-			PostType:  post.TypeText,
-			Content:   plan.textHTML,
-		}
-
-	case ".md":
-		html := plan.mdHTML
-		for _, name := range plan.localImages {
-			html = strings.ReplaceAll(html,
-				fmt.Sprintf(`src="%s"`, name),
-				fmt.Sprintf(`src="posts/%s/%s"`, id, name))
-		}
-
-		sourceDir := filepath.Dir(plan.srcPath)
-		for _, name := range plan.localImages {
-			src := filepath.Join(sourceDir, name)
-			dst := filepath.Join(postDir, name)
-			if err := copyFile(src, dst); err != nil {
-				_ = os.RemoveAll(postDir)
-				return fmt.Errorf("copy markdown asset %s: %w", name, err)
-			}
-			inboxExtras = append(inboxExtras, src)
-		}
-
-		p = &post.Post{
-			ID:        id,
-			Timestamp: now,
-			PostType:  post.TypeMarkdown,
-			Content:   html,
-			Assets:    plan.localImages,
-		}
-
-	case ".png", ".jpg", ".jpeg", ".gif":
-		if err := writeImageAsset(plan.validatedImage, postDir); err != nil {
-			_ = os.RemoveAll(postDir)
-			return err
-		}
-		src := fmt.Sprintf("posts/%s/image.jpg", id)
-		html := fmt.Sprintf(`<img src="%s" alt="" class="post-image">`, src)
-		p = &post.Post{
-			ID:        id,
-			Timestamp: now,
-			PostType:  post.TypeImage,
-			Content:   html,
-			Assets:    []string{"image.jpg"},
-		}
-
-	case ".mp3":
-		outName := filepath.Base(plan.srcPath)
-		dst := filepath.Join(postDir, outName)
-		if err := copyFile(plan.srcPath, dst); err != nil {
-			_ = os.RemoveAll(postDir)
-			return err
-		}
-		src := fmt.Sprintf("posts/%s/%s", id, outName)
-		html := fmt.Sprintf(
-			`<audio controls class="post-audio"><source src="%s" type="audio/mpeg">Your browser does not support audio.</audio>`,
-			src,
-		)
-		p = &post.Post{
-			ID:        id,
-			Timestamp: now,
-			PostType:  post.TypeAudio,
-			Content:   html,
-			Assets:    []string{outName},
-		}
+	p, inboxExtras, err := plan.builder.Commit(plan, postDir, id, now)
+	if err != nil {
+		_ = os.RemoveAll(postDir)
+		return err
 	}
 
 	if err := p.Save(postDir); err != nil {
diff --git a/internal/processor/txt.go b/internal/processor/txt.go
index 8381271..f230d37 100644
--- a/internal/processor/txt.go
+++ b/internal/processor/txt.go
@@ -6,12 +6,40 @@ import (
 	"os"
 	"regexp"
 	"strings"
+	"time"
+
+	"codeberg.org/snonux/snonux/internal/post"
 )
 
 // urlPattern matches http/https URLs in plain text.
 // Trailing sentence punctuation is stripped separately by stripURLTrailing.
 var urlPattern = regexp.MustCompile(`https?://\S+`)
 
+type txtBuilder struct{}
+
+func (txtBuilder) Plan(srcPath string, ext string) (postPlan, error) {
+	plan := postPlan{srcPath: srcPath, ext: ext}
+	html, err := processTxt(srcPath)
+	if err != nil {
+		return postPlan{}, err
+	}
+	plan.textHTML = html
+	return plan, nil
+}
+
+func (txtBuilder) Commit(plan postPlan, postDir string, id string, now time.Time) (*post.Post, []string, error) {
+	return &post.Post{
+		ID:        id,
+		Timestamp: now,
+		PostType:  post.TypeText,
+		Content:   plan.textHTML,
+	}, nil, nil
+}
+
+func init() {
+	register(".txt", txtBuilder{})
+}
+
 // processTxt reads a plain-text file and wraps each non-empty paragraph in <p> tags.
 // URLs are automatically converted to clickable <a> links.
 // Non-URL text is HTML-escaped to prevent XSS.