DB-backed incremental redesign: find tracks missing locally

Replace the JSON-file pipeline with a SQLite/SQLAlchemy store (Postgres via
DATABASE_URL) and an incremental, ID-based matcher whose goal is "which
Spotify/iTunes tracks am I missing locally".

- src/db: models (Track/MbCache/FileState/IngestState) + engine/session.
- src/ingest: upsert prefetched spotify/itunes/local (work_dir/latest-1) by
  (source, source_id); pulls ISRC from raw Spotify; preserves enrichment.
- src/enrich: MusicBrainz recording-MBID resolver. Resolves via NORMALIZED
  text search (so the same song across sources lands on one MBID + one cached
  lookup), ISRC as exact fallback. Only touches unresolved tracks, caches
  positive AND negative results, rate-limited and resumable.
- src/match: normalization + cross-source matcher with tiers ISRC > MBID >
  normalized text; reports present/missing and the match method.
- main.py: CLI (ingest / enrich / match). Gui.py: missing-tracks viewer.
- README + requirements (SQLAlchemy, musicbrainzngs); ignore *.db.

Incremental everywhere: re-running updates only what changed; enrich continues
where it stopped. Live Spotify/local backends are left intact.

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

Env.py

@@ -11,3 +11,15 @@ work_dir = Path(os.environ.get("WORK_DIR", "./work_dir/new"))
 #   export SPOTIFY_CLIENT_SECRET=...
 client_id = os.environ.get("SPOTIFY_CLIENT_ID")
 client_secret = os.environ.get("SPOTIFY_CLIENT_SECRET")
+
+# Persistent store. Defaults to a local SQLite file; point DATABASE_URL at a
+# Postgres instance (postgresql+psycopg://user:pass@host/db) to switch with no
+# code changes.
+database_url = os.environ.get("DATABASE_URL", "sqlite:///./musicindexer.db")
+
+# Folder of prefetched JSON used to (re)build the DB without hitting the network.
+prefetched_dir = Path(os.environ.get("PREFETCHED_DIR", "./work_dir/latest-1"))
+
+# Contact email sent to MusicBrainz in the User-Agent (be a good API citizen).
+mb_contact = os.environ.get("MB_CONTACT", "music-indexer@example.com")
+