Implemented plan-level and project-level locking with configurable timeouts.
Added locks table via Alembic migration storing owner_id, resource_type,
resource_id, acquired_at, and expires_at. Locks enforced in
PlanLifecycleService transitions. Support for re-entrant acquisition,
lock renewal, graceful shutdown release, and startup cleanup of expired
locks. Added diagnostics check for stale lock reporting.
ISSUES CLOSED: #327
