Add render_as_batch for SQLite and document migration rules

- Set render_as_batch=True in Alembic env.py so autogenerated
  migrations use batch mode for SQLite compatibility
- Add SQLite + Alembic section to CLAUDE.md with rules for
  avoiding circular dependency and idempotency issues

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

CLAUDE.md

@@ -258,6 +258,16 @@ Admin endpoints use `game_service.py` functions which handle both database updat
 3. Return JSON responses with appropriate status codes
 4. For admin actions affecting game state, use functions from `game_service.py` to ensure WebSocket broadcasts
 
+## SQLite + Alembic Migrations
+
+This project uses **SQLite** as its database. SQLite has limited ALTER TABLE support, so Alembic uses **batch mode** (`render_as_batch=True` in `migrations/env.py`) to handle schema changes by recreating tables behind the scenes.
+
+**When writing or generating migrations, you MUST:**
+- **Always test migrations can actually run** against SQLite — don't just generate and commit blindly.
+- **Watch for circular dependency errors** in batch mode. If a table has foreign keys (especially self-referential or mutual FKs like `games` ↔ `teams`), splitting a single `batch_alter_table` into multiple calls (e.g., one for column adds, one for FK creation) avoids `CircularDependencyError`.
+- **Make migrations idempotent when possible** — check if columns/tables already exist before adding them, especially if production DB state may have drifted from Alembic history. Use `PRAGMA table_info('table_name')` to inspect existing columns.
+- **Never combine column adds/drops with FK changes** in a single `batch_alter_table` block on tables that have existing foreign keys.
+
 ## Important Notes
 
 - **Port 5000 conflict**: macOS AirPlay Receiver uses port 5000 by default. Use `PORT=5001` when running the backend.