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>

src/db/database.py

@@ -0,0 +1,35 @@
+"""Engine/session helpers. The DB URL comes from Env.database_url so the same
+code runs on SQLite (default) or Postgres by changing one env var."""
+from contextlib import contextmanager
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from Env import database_url
+from src.db.models import Base
+
+_engine = create_engine(database_url, future=True)
+SessionLocal = sessionmaker(bind=_engine, future=True, expire_on_commit=False)
+
+
+def get_engine():
+    return _engine
+
+
+def init_db():
+    """Create tables if they don't exist (safe to call every run)."""
+    Base.metadata.create_all(_engine)
+
+
+@contextmanager
+def session_scope():
+    """Transactional session: commit on success, roll back on error."""
+    session = SessionLocal()
+    try:
+        yield session
+        session.commit()
+    except Exception:
+        session.rollback()
+        raise
+    finally:
+        session.close()