feat: auto-scan soundfont for Synth instrument list

Background daemon (30s interval) tracks scanned files via
sf_scan_state.json (size+mtime). Only inspects new/changed SFs.
Catalog served from scanner cache, no full re-scan per request.
Upload/delete trigger immediate re-scan.

app/main.py

@@ -15,6 +15,7 @@ from app.api.v1.ai_proxy import router as ai_proxy_router
 from app.api.v1.plugins import router as plugins_router
 from app.core.auth import seed_admin
 from app.core.soundfont_converter import SoundFontConverter
+from app.core.soundfont_scanner import SoundFontAutoScanner
 
 # Ensure storage directories exist
 os.makedirs(settings.UPLOADS_DIR, exist_ok=True)
@@ -65,6 +66,11 @@ async def startup_convert_soundfonts():
             print(f"[Startup] SoundFont conversion error: {e}")
     threading.Thread(target=_run, daemon=True).start()
 
+@app.on_event("startup")
+async def startup_sf_scanner():
+    scanner = SoundFontAutoScanner()
+    scanner.start_background(interval=30)
+
 @app.get("/", response_class=HTMLResponse)
 async def get_index():
     index_path = os.path.join(settings.TEMPLATES_DIR, "index.html")