Incremental Spotify ingest + --no-enrich

- spotify_raw: skip re-ingesting when the cache files are unchanged (signature
  in ingest_state), matching the iTunes/local incremental behaviour — so a
  re-run with no changes touches nothing.
- fetch --no-enrich: skip MusicBrainz enrichment and rely on MBIDs already on
  tracks (e.g. local Picard tags).

Verified on the real library: 1st run ~9s, unchanged re-run ~0.3s with all
three sources skipping; with enrichment off, 1164 Spotify tracks still match
locally by ISRC/MBID from tags.

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

old-work-dir/first-fetch/user_data.json

@@ -0,0 +1,26 @@
+{
+    "display_name": "ilusha",
+    "external_urls": {
+        "spotify": "https://open.spotify.com/user/31uszk7zb6zdzrzst4q3rvkfrfz4"
+    },
+    "followers": {
+        "href": null,
+        "total": 8
+    },
+    "href": "https://api.spotify.com/v1/users/31uszk7zb6zdzrzst4q3rvkfrfz4",
+    "id": "31uszk7zb6zdzrzst4q3rvkfrfz4",
+    "images": [
+        {
+            "url": "https://i.scdn.co/image/ab6775700000ee85b4a21a7f0c35258b3fa4b79d",
+            "height": 300,
+            "width": 300
+        },
+        {
+            "url": "https://i.scdn.co/image/ab67757000003b82b4a21a7f0c35258b3fa4b79d",
+            "height": 64,
+            "width": 64
+        }
+    ],
+    "type": "user",
+    "uri": "spotify:user:31uszk7zb6zdzrzst4q3rvkfrfz4"
+}