Add feedback system: SQLite DB, rating tools, reference search

- feedback.py: SQLite-backed FeedbackDB (add, rate, search, export)
- server.py: auto-save to DB on generate, 4 new MCP tools:
  - rate_sprite: rate 1-5 stars with feedback
  - get_reference_sprites: find high-rated similar sprites
  - list_sprites: list all/unrated/top
  - db_stats: database statistics
- Updated README with feedback tool docs

.gitignore

@@ -13,6 +13,10 @@ build/
 # Output (generated sprites)
 output/
 
+# Feedback database
+feedback.db
+feedback_dataset.jsonl
+
 # IDE
 .idea/
 .vscode/