feat(scanner): fix ABS sidecar series parsing; three-way series mapping; smart genre; language codes; title-derived series

Phase 2a of the metadata overhaul (plan §2.1-2.3, §2.5-2.9).

The headline bug: ABS writes sidecar series as an array of STRINGS
(["The Witcher #3", ...]); the parser only accepted {series, sequence}
OBJECT arrays, so every ABS-matched book silently lost its series (found
live: 9 sidecars, zero series ingested; only folder-structure Malazan
survived). parseSeriesCandidates now normalizes string arrays, object
arrays and Libation plain strings.

Three-way series assignment (plan §2.2):
- primary bibliographic series -> series/series_number columns
- additional real series (Drizzt: Icewind Dale #1 = Legend of Drizzt #4)
  -> alternate_info JSONB, comic-established shape
- ordering constructs ('The Witcher (Reading Order)') -> PlaylistSeries
  candidates; NEVER the series column. Detection is explicit-marker-only
  ((reading order|chronological|publication order|in order|timeline) as
  parenthetical/suffix) — precision over recall; a series merely
  containing 'timeline' mid-name stays a series.

Sequence policy: leading-number extraction ('Book 2' -> 2, '2,
Dramatized Adaptation' -> 2, '1a' -> 1); lossy extractions preserve the
raw text in alternate_info.series_sequence_raw.

Smart genre (deterministic lexicon): compound Audible blobs split
('Epic, Action & Adventure, Military, Sword & Sorcery' -> 4 candidates),
alias-normalized, format noise demoted ('Audiobook' never wins),
most-specific canonical genre takes the genre column, all candidates
stay in tags.

Language: display names normalize to ISO 639-1 on ingest; ROOT-CAUSED
the update-path drop — CreateMediaItem call site never set Language
(explicit NULL over the column default), the update path did; create
now carries it.

Title-derived series fallback (§2.7): 'Baptism of Fire: The Witcher,
Book 3' 2-level folders split conservatively (only when the post-colon
tail matches ', Book N'), subtitle parsed for series+number, blanks-only
precedence, matching-name number fill, zero display mutation. Works
with no sidecar and no network.

Ingest fixes: subtitle/explicit land in their columns (audio tag
subtitle stops joining into title); full authors[] array (was [0]-
only); sidecar descriptions sanitized via bluemonday UGC policy
(vendored).

metadata_overrides: verified scan-update path already applies
ApplyMetadataOverrides; subtitle/explicit/authors covered by the Phase 1
override adaptation.

Tests: new series_parsing_test.go (string-array sidecar e2e, Drizzt
alternate_info, lossy raw preservation, playlist classification,
title-derived fallback incl. no-mutation + blanks-only, genre lexicon,
language map); audio-tag subtitle pin updated to column behavior;
calibre/ABS 2.5-series_index pins widened per float change (changelog'd
in phase 1).

Deferred to 2b: auto-created playlist collections (§2.4) — settings
toggle, collection upsert, ownership.

internal/services/media_scanner_audio.go

@@ -408,12 +408,10 @@ func applyAudioTagFallbacks(metadata *MediaMetadata, report *ffprobeReport) {
 			metadata.Title = album
 		}
 	}
-	// Subtitle joins the title Calibre-style, mirroring the ABS sidecar
-	// path — there is no separate subtitle column. Skip when the title
-	// already contains it (tools sometimes pre-join).
-	if subtitle := report.tag("----:com.apple.itunes:subtitle", "subtitle"); subtitle != "" && metadata.Title != "" &&
-		!strings.Contains(strings.ToLower(metadata.Title), strings.ToLower(subtitle)) {
-		metadata.Title = metadata.Title + ": " + subtitle
+	// Subtitle is a first-class column (A2b) — no more Calibre-style title
+	// joining; the tool-provided subtitle lands in its own column.
+	if subtitle := report.tag("----:com.apple.itunes:subtitle", "subtitle"); subtitle != "" {
+		metadata.Subtitle = subtitle
 	}
 	if metadata.Author == "" {
 		if artist := report.tag("album_artist"); artist != "" {