Incremental live Spotify fetch (snapshot/watermark)

The live fetch no longer re-pulls everything each run:
- playlists: re-fetch a playlist's tracks only when its snapshot_id changed
  (new/deleted playlists handled); state in spotify_playlist table.
- liked songs: page newest-first, stop at the newest added_at already seen
  (watermark in ingest_state).
- cover art: download only when the file is missing or the URL changed.

New src/ingest/spotify_fetch.py drives this against the low-level
SpotifyWebAPI client and upserts straight to the DB; fetch uses it when creds
are set (falls back to the cached-JSON ingest otherwise). `fetch --spotify-full`
forces a complete resync. Shared upsert helper extracted into spotify_raw.

Verified with a mocked API: unchanged re-run does 0 playlist-track fetches and
one liked page; a snapshot change re-fetches exactly that playlist.

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

main.py

@@ -19,7 +19,8 @@ def cmd_fetch(args):
     _setup_logging()
     from src.workflows import run_fetch
 
-    run_fetch(enrich_limit=args.enrich_limit, enrich=not args.no_enrich)
+    run_fetch(enrich_limit=args.enrich_limit, enrich=not args.no_enrich,
+              spotify_full=args.spotify_full)
 
 
 def cmd_display(args):
@@ -56,6 +57,8 @@ def build_parser():
                        help="cap MBID lookups this run (default: resolve all)")
     fetch.add_argument("--no-enrich", action="store_true",
                        help="skip MusicBrainz enrichment; use MBIDs already on tracks")
+    fetch.add_argument("--spotify-full", action="store_true",
+                       help="force a full Spotify resync (ignore snapshot/watermark)")
 
     sub.add_parser("display", help="open the Streamlit GUI")