Add feedback loop: SQLite-backed rating system + MCP tools + GUI Training tab

New crate: soundgen-feedback
- FeedbackDB with SQLite (rusqlite, bundled)
- CRUD: add, update_rating, get_all, get_unrated, top_rated, delete
- search_similar: keyword-based similarity for few-shot reference
- export_jsonl: export rated examples for fine-tuning
- 9 tests

MCP server: 5 tools (was 3)
- generate_batch: generate multiple sounds by name, store in DB
- get_reference_sounds: search DB for highly-rated similar sounds
- render_sound: now returns reference examples from DB in response
- --db flag to specify feedback database path

GUI: Training tab
- Open/create feedback DB (file dialog)
- Generate batch dialog (enter names, output dir)
- Sound list with star rating (1-5), feedback text, play button
- Export dataset as JSONL
- Stats display (total/rated/avg rating)

OpenCode config updated with --db feedback.db path

crates/soundgen-feedback/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "soundgen-feedback"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+soundgen-core.workspace = true
+soundgen-fmt.workspace = true
+rusqlite.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+uuid.workspace = true