test(rls): add multi-tenant isolation integration tests
- 6 comprehensive RLS tests: complete isolation, no context, insert protection, concurrent requests, cross-tenant spoof, interceptor verification
- Uses Testcontainers PostgreSQL + Dapper for raw SQL validation
- Parallel safety test: 50 concurrent requests with ConcurrentBag
- Build passes: 0 errors (6 expected BouncyCastle warnings)
- Evidence: task-13-rls-isolation.txt (21KB), task-13-concurrent-safety.txt
- Learnings: RLS testing patterns, SET LOCAL vs SET, concurrent testing with Task.WhenAll
Task 13 complete. Wave 3: 1/5 tasks done.
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