feat: add RLS Policy Playground API endpoints

Adds new API endpoints for simulating and debugging Row Level Security policies:

- GET /rls-playground/roles - List available database roles
- GET /rls-playground/tables?schema=X - List tables with RLS status
- GET /rls-playground/policies/:schema/:table - Get policies for a table
- GET /rls-playground/rls-status/:schema/:table - Check if RLS is enabled
- POST /rls-playground/simulate - Run policy simulation with custom context
- POST /rls-playground/evaluate-expression - Test single policy expression

The simulation endpoint allows users to:
- Set a database role (anon, authenticated, service_role, custom)
- Provide JWT claims to simulate auth.jwt()
- See which rows are accessible under the simulated context
- Get policy-by-policy evaluation results for debugging

All simulations run in rolled-back transactions for safety.

Author: jonathan-fulton