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>

migrations/env.py

@@ -100,6 +100,7 @@ def run_migrations_online():
         context.configure(
             connection=connection,
             target_metadata=get_metadata(),
+            render_as_batch=True,
             **conf_args
         )