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/core/soundfont_inspector.py

@@ -104,8 +104,8 @@ class SoundFontInspector:
             return self._catalog_cache
         return self.generate_full_catalog()
 
-    def get_condensed_catalog_summary(self) -> dict:
-        catalog = self.get_catalog()
+    def get_condensed_catalog_summary(self, catalog: dict = None) -> dict:
+        catalog = catalog if catalog is not None else self.get_catalog()
         condensed = {}
         for sf_id, sf_info in catalog.items():
             instruments = sf_info.get("instruments", [])