feat(audiobooks): Batch A — foundation, m4b ingest, playback sync, web player

Phase 1 (foundation): 'audiobooks' library type seeded (schema + DB);
audio_metadata JSONB on media_items (the kobo_metadata precedent —
only audio paths read it); reading_progress.audio_offset_seconds for
the honest {chapter, offset} locator; extensions/mimes registered for
ALL breadth formats (m4b/m4a/mp3/flac — deliberate per the DOC/LIT
doctrine, m4b scans richly first); classifyFormatGroup 'audiobook'
case (gotcha 39 recomputes on rescan); Go mime table extended at boot
so media elements accept the streams; ffmpeg/ffprobe in the image
(8.1.2 — also the xHE-AAC identification tripwire).

Phase 2 (ingest): sidecar-first (Libation/ABS metadata.json —
narrators/ASIN/curated chapters), ffprobe fallback/filler (duration,
bitrate, codec/profile, embedded chapters). Fixed three real
parser brittlenesses the Libation sidecar exposed: series accepts
string OR array-of-objects, publishedYear accepts number OR string,
ffprobe chapter timings accept number OR string (flexFloat). Folder-
structure fallback now runs on the UPDATE path too (rescans were
clearing sidecar-absent series/author); audio facts re-derive on
every rescan (gotcha 39).

Phase 3 (playback + sync): GET /api/media-items/:id/chapters (the
format-abstraction list); streaming rides the existing cookie-authed
Range /uploads path; PUT progress carries audio_offset_seconds +
elapsed_seconds (honest wall-clock playing time) — the reading-
history capture prefers player-reported elapsed over the inter-push
delta, same 2h ceiling; reading_mode='audio' marks the row.

Phase 4 (web player): dedicated audio reader page (foliate not
involved) — cover, chapter title, whole-book scrubber, skip ± (chips
1/5/10/15/30s, default 10s), speed (0.75×–2×), chapter list with
jump-to, Media Session lockscreen/notification transport, periodic
playing-only progress pushes, position restore on open.

Verified live: ingest of the Libation sample (Gardens of the Moon —
title/author/series/cover/date from sidecar+folders, 34 chapters,
26h duration, aac/LC); chapters endpoint; the reader route serving
the player; playback with position restore (0:57 round-trip);
progress rows in audio mode; wall-clock session rows in
reading_history.

internal/services/library_service.go

@@ -24,15 +24,20 @@ func NewLibraryService(db *database.Queries) *LibraryService {
 
 // Library type definitions and file extensions
 const (
-	LibraryTypeEbooks = "ebooks"
-	LibraryTypeComics = "comics"
-	LibraryTypeManga  = "manga"
+	LibraryTypeEbooks     = "ebooks"
+	LibraryTypeComics     = "comics"
+	LibraryTypeManga      = "manga"
+	LibraryTypeAudiobooks = "audiobooks"
 )
 
 var AllowedExtensions = map[string][]string{
 	LibraryTypeEbooks: {".epub", ".pdf", ".mobi", ".azw", ".azw3", ".txt", ".rtf", ".docx", ".fb2", ".pdb"},
 	LibraryTypeComics: {".cbz", ".cbr", ".cb7", ".cbt", ".epub", ".pdf"},
 	LibraryTypeManga:  {".cbz", ".cbr", ".epub", ".pdf", ".png", ".jpg", ".jpeg", ".gif", ".bmp", ".webp", ".avif", ".tiff", ".tif"},
+	// All four decided breadth formats registered from day one (the
+	// DOC/LIT doctrine: extensions are a deliberate list); only m4b
+	// scans richly in Batch A — the chapter abstraction carries the rest.
+	LibraryTypeAudiobooks: {".m4b", ".m4a", ".mp3", ".flac"},
 }
 
 var MimeTypes = map[string]string{
@@ -62,6 +67,12 @@ var MimeTypes = map[string]string{
 	".docx": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
 	".fb2":  "application/x-fictionbook+xml",
 	".pdb":  "application/vnd.palm",
+	// Audiobooks — audio/mp4 for the MP4-family containers is what
+	// every platform decoder expects from an <audio>/ExoPlayer source.
+	".m4b":  "audio/mp4",
+	".m4a":  "audio/mp4",
+	".mp3":  "audio/mpeg",
+	".flac": "audio/flac",
 }
 
 // GetLibraryTypes retrieves all available library types