Add fetch_music.py (Jamendo API) + expand genre keywords

- Downloads instrumental tracks from Jamendo by genre (jazz, lofi, blues, ambient, etc.)
- Filters: no vocals, 60-300s, sorted by popularity
- Saves to music/ with genre tags, tracks attribution
- Add genre keywords: ambient, chill, acoustic, classical, country, electronic

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/main.py

@@ -8780,6 +8780,15 @@ GENRE_KEYWORDS = {
     "irish": "Irish",
     "ireland": "Irish",
     "patricks": "Irish",
+    "ambient": "Ambient",
+    "chill": "Chill",
+    "acoustic": "Acoustic",
+    "classical": "Classical",
+    "piano": "Classical",
+    "country": "Country",
+    "western": "Country",
+    "electronic": "Electronic",
+    "synth": "Electronic",
 }