feat: SF3 conversion + SpessaSynth client player

Server:
- soundfont_converter.py: Python SF2->SF3 via ffmpeg Ogg compression
- batch_convert_all runs on startup (daemon thread)
- GET /soundfonts/download/{sf_id} serves SF3 with SF2 fallback
- Dockerfile: add fluidsynth, vorbis-tools

Client:
- soundfontStorage.js: IndexedDB cache for SF3 buffers
- soundfontPlayer.js: dual-mode (SpessaSynth + oscillator fallback)
- app.jsx: init SpessaSynth, loadSF on instrument select
- index.html: SpessaSynth CDN import + storage script tag

Compression: DSK 11M->1.1M (90%), SGM 529M->18M (97%)

app/main.py

@@ -1,4 +1,4 @@
-import os
+import os, threading
 from fastapi import FastAPI
 from fastapi.responses import HTMLResponse
 from fastapi.staticfiles import StaticFiles
@@ -14,6 +14,7 @@ from app.api.v1.user_config import router as user_config_router
 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
 
 # Ensure storage directories exist
 os.makedirs(settings.UPLOADS_DIR, exist_ok=True)
@@ -55,6 +56,15 @@ app.include_router(plugins_router, prefix="/api/v1/plugins", tags=["plugins"])
 async def startup_seed_admin():
     seed_admin()
 
+@app.on_event("startup")
+async def startup_convert_soundfonts():
+    def _run():
+        try:
+            SoundFontConverter().batch_convert_all()
+        except Exception as e:
+            print(f"[Startup] SoundFont conversion error: {e}")
+    threading.Thread(target=_run, daemon=True).start()
+
 @app.get("/", response_class=HTMLResponse)
 async def get_index():
     index_path = os.path.join(settings.TEMPLATES_DIR, "index.html")