feat(media): images and recordings in backups, exports and imports (M6)

- Backup ZIP (download and automatic per-user server backups) carries the
  files of the user's own notes as media/<id><ext>; restoring creates
  them with new IDs and rewrites the blocks.
- Markdown archive puts them under attachments/ and embeds them relative
  to each note (![caption](../attachments/…)); re-importing turns them
  back into image/audio blocks.
- Obsidian import takes images and audio a note embeds on its own
  (![[file.png]], ![](path/file.png)); unused or invalid files appear as
  skipped in the preview instead of vanishing.
- Format detection and file paths moved into storage for this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/media.go

@@ -23,35 +23,9 @@ import (
 
 const mediaGrace = 24 * time.Hour // uploads not yet placed in a block are kept this long
 
-// Accepted formats (detected from the content, not from the name): file extension and kind
-var mediaTypes = map[string]struct{ ext, kind string }{
-	"image/jpeg":      {".jpg", "image"},
-	"image/png":       {".png", "image"},
-	"image/gif":       {".gif", "image"},
-	"image/webp":      {".webp", "image"},
-	"audio/webm":      {".webm", "audio"},
-	"video/webm":      {".webm", "audio"}, // Chrome's audio-only recordings are detected as video/webm
-	"audio/ogg":       {".ogg", "audio"},
-	"application/ogg": {".ogg", "audio"},
-	"audio/mpeg":      {".mp3", "audio"},
-	"audio/wave":      {".wav", "audio"},
-	"audio/wav":       {".wav", "audio"},
-	"audio/mp4":       {".m4a", "audio"},
-	"video/mp4":       {".m4a", "audio"}, // Safari records MP4/AAC
-	"audio/aac":       {".aac", "audio"},
-}
+func (s *Server) mediaRoot() string { return s.store.MediaRoot() }
 
-func (s *Server) mediaRoot() string {
-	return filepath.Join(filepath.Dir(s.cfg.DatabasePath), "media")
-}
-
-func (s *Server) mediaPath(m *storage.Media) string {
-	ext := ".bin"
-	if t, ok := mediaTypes[m.Mime]; ok {
-		ext = t.ext
-	}
-	return filepath.Join(s.mediaRoot(), filepath.Base(m.DocID), filepath.Base(m.ID)+ext)
-}
+func (s *Server) mediaPath(m *storage.Media) string { return s.store.MediaFilePath(m) }
 
 // mediaLimit is the largest allowed file of a kind in bytes.
 func (s *Server) mediaLimit(kind string) int64 {
@@ -65,37 +39,7 @@ func (s *Server) mediaLimit(kind string) int64 {
 	return int64(mb) << 20
 }
 
-// detectMedia recognises the format from the first bytes. Browsers send audio as e.g.
-// "audio/webm;codecs=opus"; that name is kept when the content looks like a container we accept.
-func detectMedia(data []byte, declared string) (mime, kind string, ok bool) {
-	sniffed := http.DetectContentType(data)
-	if i := strings.IndexByte(sniffed, ';'); i >= 0 {
-		sniffed = sniffed[:i]
-	}
-	// Go does not know WebP and M4A by name; check their signatures directly
-	switch {
-	case len(data) >= 12 && string(data[0:4]) == "RIFF" && string(data[8:12]) == "WEBP":
-		sniffed = "image/webp"
-	case len(data) >= 12 && string(data[4:8]) == "ftyp":
-		sniffed = "video/mp4"
-	}
-	t, ok := mediaTypes[sniffed]
-	if !ok {
-		return "", "", false
-	}
-	if t.kind == "audio" {
-		if d := strings.ToLower(strings.TrimSpace(strings.SplitN(declared, ";", 2)[0])); strings.HasPrefix(d, "audio/") {
-			if dt, known := mediaTypes[d]; known && dt.kind == "audio" {
-				sniffed = d
-			}
-		} else if sniffed == "video/webm" {
-			sniffed = "audio/webm"
-		} else if sniffed == "video/mp4" {
-			sniffed = "audio/mp4"
-		}
-	}
-	return sniffed, t.kind, true
-}
+var detectMedia = storage.DetectMedia
 
 func (s *Server) canEditDoc(userID, docID string) bool {
 	role := s.store.DocRole(userID, docID)