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>

README.md

@@ -78,8 +78,17 @@ python main.py display    # open the GUI to see what you're missing
 **`fetch`** pulls Spotify, scans the local library, parses the iTunes export,
 resolves MBIDs, and matches — logging progress at each step. The **first run is
 slow** (full scan + MusicBrainz lookups at ~1 req/s); **later runs are quick**
-because only changed files, the iTunes export if it changed, and still-unresolved
-MBIDs are touched. Useful flags/helpers:
+because each source only touches what changed:
+
+- **Spotify** — playlists are re-fetched only when their `snapshot_id` changes;
+  liked songs are paged newest-first and stop at the newest one already stored;
+  cover art downloads only when missing or its URL changed (`--spotify-full`
+  forces a complete resync).
+- **Local** — only files whose mtime/size changed; deleted files are pruned.
+- **iTunes** — skipped entirely if the export's mtime is unchanged.
+- **MBID enrichment** — only still-unresolved tracks.
+
+Useful flags/helpers:
 
 ```bash
 python main.py fetch --enrich-limit 500   # cap MBID lookups this run