refactor: rename backend/frontend dirs and remove NovelWriter submodule

- Rename qwen3-tts-backend → canto-backend
- Rename qwen3-tts-frontend → canto-frontend
- Remove NovelWriter embedded repo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

canto-backend/db/migrate_add_local_permission.py

@@ -0,0 +1,17 @@
+from sqlalchemy import create_engine, text
+from core.config import settings
+
+def migrate():
+    engine = create_engine(settings.DATABASE_URL)
+    with engine.connect() as conn:
+        conn.execute(text(
+            "ALTER TABLE users ADD COLUMN can_use_local_model BOOLEAN DEFAULT 0 NOT NULL"
+        ))
+        conn.execute(text(
+            "UPDATE users SET can_use_local_model = 1 WHERE is_superuser = 1"
+        ))
+        conn.commit()
+    print("Migration completed: Added can_use_local_model column")
+
+if __name__ == "__main__":
+    migrate()