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/api/v1/plugins.py

@@ -8,6 +8,7 @@ from app.core.vst_engine import PluginManager, HAS_PEDALBOARD, HAS_PYFLUIDSYNTH
 from app.core.render_engine import PythonRenderEngine
 from app.core.soundfont_inspector import SoundFontInspector
 from app.core.soundfont_converter import SoundFontConverter
+from app.core.soundfont_scanner import SoundFontAutoScanner
 from app.api.v1.auth import get_current_user
 
 router = APIRouter()
@@ -18,6 +19,7 @@ os.makedirs(UPLOAD_SF_DIR, exist_ok=True)
 SYSTEM_SF_DIR = "/opt/daw_engine/soundfonts"
 
 _inspector = None
+_scanner = None
 
 def get_inspector():
     global _inspector
@@ -25,6 +27,13 @@ def get_inspector():
         _inspector = SoundFontInspector(system_sf_dir=SYSTEM_SF_DIR, upload_sf_dir=UPLOAD_SF_DIR)
     return _inspector
 
+def get_scanner():
+    global _scanner
+    if _scanner is None:
+        _scanner = SoundFontAutoScanner(system_sf_dir=SYSTEM_SF_DIR, upload_sf_dir=UPLOAD_SF_DIR)
+        _scanner.scan_once()
+    return _scanner
+
 
 @router.get("/available")
 async def list_plugins(current_user: dict = Depends(get_current_user)):
@@ -50,9 +59,10 @@ async def list_default_soundfonts():
 
 @router.get("/soundfonts/catalog")
 async def soundfont_catalog(current_user: dict = Depends(get_current_user)):
+    scanner = get_scanner()
+    full_catalog = scanner.get_catalog()
     inspector = get_inspector()
-    full_catalog = inspector.get_catalog()
-    condensed_catalog = inspector.get_condensed_catalog_summary()
+    condensed_catalog = inspector.get_condensed_catalog_summary(full_catalog)
     return {"full_catalog": full_catalog, "condensed_catalog": condensed_catalog}
 
 
@@ -91,11 +101,11 @@ async def upload_soundfont(
         import json
         json.dump({"original_name": file.filename, "uuid": file_uuid, "file": file_id}, f)
 
-    inspector = get_inspector()
-    inspector.invalidate_catalog_cache()
+    scanner = get_scanner()
     if background_tasks:
-        background_tasks.add_task(inspector.generate_full_catalog)
-
+        background_tasks.add_task(scanner.scan_once)
+    else:
+        scanner.scan_once()
     return {"id": file_id, "name": file.filename, "path": dest_path, "size_bytes": len(contents)}
 
 
@@ -122,8 +132,7 @@ async def delete_soundfont(sf_id: str, current_user: dict = Depends(get_current_
             break
     if not deleted:
         raise HTTPException(status_code=404, detail="SoundFont not found")
-    inspector = get_inspector()
-    inspector.invalidate_catalog_cache()
+    get_scanner().scan_once()
     return {"deleted": True, "sf_id": sf_id}