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/router/reader.go

@@ -83,7 +83,10 @@ func registerReaderRoutes(cfg *Config) {
 			MediaItemID:      mediaUUID.String(),
 			Title:            mediaItem.Title,
 			Author:           textToString(mediaItem.Author),
-			CoverImagePath:   textToString(mediaItem.CoverImagePath),
+			// Sidecar covers store a raw library-relative path; resolve to
+			// the cookie-authed /uploads URL (idempotent for items whose
+			// stored value is already a servable path).
+			CoverImagePath:   utils.ResolveMediaURL(mediaItem.LibraryID, mediaItem.CoverImagePath),
 			LibraryType:      mediaItem.FormatGroup,
 			MimeType:         textToString(mediaItem.MimeType),
 			FilePath:         mediaItem.FilePath,
@@ -100,7 +103,13 @@ func registerReaderRoutes(cfg *Config) {
 		}
 		// Render template
 		var buf bytes.Buffer
-		err = templates.Reader(user, metadata).Render(c.Request().Context(), &buf)
+		// Audiobooks get the audio player page — the foliate document
+		// engine is deliberately not involved (handoff item 3 Batch A).
+		if mediaItem.FormatGroup == "audiobook" {
+			err = templates.AudioReader(user, metadata).Render(c.Request().Context(), &buf)
+		} else {
+			err = templates.Reader(user, metadata).Render(c.Request().Context(), &buf)
+		}
 		if err != nil {
 			return renderErrorPage(c, "Error rendering reader", "render_error")
 		}