processor: refactor to two-phase commit for inbox processing

Introduce postPlan to capture everything validated in Phase 1 before
any filesystem mutation occurs. Phase 1 scans the inbox, validates all
source files (parses text, markdown, image, audio), checks markdown
image claims for conflicts, and collects a plan per item. Phase 2
commits mutations only after every plan is validated: creates post
directories, writes assets, persists post.json, and removes sources.

If Phase 1 fails (e.g. unsupported file, missing markdown image, claim
conflict), no mutations occur and the inbox is left untouched. Roll
back the partial post directory if a mutation fails during commit.

Also refactor image and audio sub-processors into validation-only and
write-only parts (validateImage/writeImageAsset, validateAudio/copyFile)
so that Phase 1 is strictly read-only.

All existing tests pass.

1 files changed, 6 insertions, 18 deletions

diff --git a/internal/processor/audio.go b/internal/processor/audio.go
index 98aedcf..68938cf 100644
--- a/internal/processor/audio.go
+++ b/internal/processor/audio.go
@@ -4,27 +4,15 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"path/filepath"
 )
 
-// processAudio copies an .mp3 file into destDir and returns an HTML <audio> snippet.
-// The audio element has controls enabled so visitors can play it inline.
-func processAudio(srcPath, destDir, postID string) (filename, htmlContent string, err error) {
-	outName := filepath.Base(srcPath)
-	outPath := filepath.Join(destDir, outName)
-
-	if err := copyFile(srcPath, outPath); err != nil {
-		return "", "", err
+// validateAudio confirms the audio source file exists and is readable.
+func validateAudio(srcPath string) error {
+	f, err := os.Open(srcPath)
+	if err != nil {
+		return fmt.Errorf("open audio %s: %w", srcPath, err)
 	}
-
-	// The src attribute is relative to the site root.
-	src := fmt.Sprintf("posts/%s/%s", postID, outName)
-	html := fmt.Sprintf(
-		`<audio controls class="post-audio"><source src="%s" type="audio/mpeg">Your browser does not support audio.</audio>`,
-		src,
-	)
-
-	return outName, html, nil
+	return f.Close()
 }
 
 // copyFile copies the file at src to dst, creating dst if it does not exist.