feat(enrichment): record status/errors and trust high-confidence AcoustID

Two related gaps surfaced from "uploaded a track, nothing changed / no status":

- A track could stay stuck on `pending` forever (an unexpected worker error
  rolled back the run without recording anything), and `failed` carried no
  reason. Add `tracks.metadata_error` + `tracks.enriched_at` (migration), stamp
  the outcome in apply_enrichment, add TrackRepository.mark_enrichment_failed,
  wrap enrich_task to persist crashes as `failed` in a fresh session, and emit a
  human-readable no-match reason. Expose metadata_error/enriched_at in TrackOut.

- The tag-first merge let junk embedded tags (e.g. "Music Track"/"Sound_13958")
  override even a 0.99-confidence AcoustID match. Add acoustid_trust_score
  (default 0.85): above it the acoustic identity wins for title/artist/album/
  year, tags are fallback; below it, tag-first as before.

Add a license-free real-file fixture (Scarlet Fire / Otis McDonald) whose junk
tags AcoustID overrides, with an always-on tag-reader test plus fpcalc/AcoustID/
network-gated identity + full-pipeline tests (skip on host, run in the container).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/schemas/track.py

@@ -17,6 +17,8 @@ class TrackOut(BaseModel):
     file_format: str
     file_size: int
     metadata_status: str
+    metadata_error: str | None
+    enriched_at: dt.datetime | None
     source: str
     has_cover: bool
     created_at: dt.datetime

app/api/v1/tracks.py

@@ -42,6 +42,8 @@ async def _build_track_out(
             file_format=t.file_format,
             file_size=t.file_size,
             metadata_status=t.metadata_status,
+            metadata_error=t.metadata_error,
+            enriched_at=t.enriched_at,
             source=t.source,
             has_cover=bool(t.album_id and albums.get(t.album_id) and albums[t.album_id].cover_path),
             created_at=t.created_at,