feat: add SoundFont inspection engine + AI instrument schema

- SoundFontInspector (sf2utils) scans .sf2, generates full/condensed catalog
- GET /api/v1/plugins/soundfonts/catalog with lazy init + cache invalidation
- AI tool generate_multitrack_midi now requires soundfont_id/bank/program
- Condensed catalog auto-injected into AI system prompt with bank rules
- Server render: FluidSynth program_select uses bank/program + channel routing (drums→ch9)
- VST3 pedalboard path inserts CC0 bank select + program change before notes
- DecentSamplerManager loads .dspreset with CWD fix for relative sample paths
- Pianobook render branch in render_engine.py
- Client SonicSF: controllerChange, programChange, applyAITrackInstrument
- Post-AI track creation applies instrument via applyAITrackInstrument
- Background cache rescan on .sf2 upload, frontend re-fetches catalog
- libcurl4 + VST3 dirs in Dockerfile

.gitignore

@@ -18,6 +18,10 @@ app/storage/processed/*
 !app/storage/uploads/.gitkeep
 !app/storage/processed/.gitkeep
 .DS_Store
+
+# VST3 and sample library directories (proprietary binaries)
+vst_plugins/
+samples/
 .idea/
 .vscode/
 *.log