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.

.sisyphus/evidence/task-10-build.txt

@@ -0,0 +1,24 @@
+$ next build
+▲ Next.js 16.1.6 (Turbopack)
+
+⚠ The "middleware" file convention is deprecated. Please use "proxy" instead. Learn more: https://nextjs.org/docs/messages/middleware-to-proxy
+  Creating an optimized production build ...
+✓ Compiled successfully in 1604.2ms
+  Running TypeScript ...
+  Collecting page data using 11 workers ...
+  Generating static pages using 11 workers (0/4) ...
+  Generating static pages using 11 workers (1/4) 
+  Generating static pages using 11 workers (2/4) 
+  Generating static pages using 11 workers (3/4) 
+✓ Generating static pages using 11 workers (4/4) in 147.7ms
+  Finalizing page optimization ...
+
+Route (app)
+┌ ○ /
+└ ○ /_not-found
+
+
+ƒ Proxy (Middleware)
+
+○  (Static)  prerendered as static content
+