Add Kubernetes liveness/readiness probes and middleware order validation

- Split /health into /health/live (liveness) and /health/ready (readiness)
  following Kubernetes conventions. Combined /health retained for backward
  compatibility with existing Docker HEALTHCHECK definitions.
- Add ReadyCheck and ReadyResponse models for structured readiness output.
- Add _assert_middleware_order() startup check enforcing:
  RateLimit → Csrf → CorrelationId middleware chain.
- Register CorrelationIdMiddleware, CsrfMiddleware, RateLimitMiddleware
  in create_app() with documented required order (reverse of processing).
- Add correlation.py, csrf.py, rate_limit.py middleware modules.
- Add health probe tests in test_health_probes.py.
- Update test_main.py with middleware order assertion tests.
- Update frontend useFetchData hook tests.
- Docs: update Deployment.md with Kubernetes probe config examples.

backend/app/middleware/correlation.py

@@ -11,6 +11,18 @@ Correlation IDs flow through the request lifecycle:
 3. Middleware stores in structlog.contextvars
 4. All log entries include the correlation ID automatically
 5. Error responses include the correlation ID for client-side correlation
+
+Processing order
+-----------------
+This middleware must be the outermost in the security-critical chain so it
+executes first on incoming requests (outermost = first to see request,
+last to see response). In the required chain:
+
+    CorrelationIdMiddleware → CsrfMiddleware → RateLimitMiddleware
+
+The registration order in ``main.py`` must be:
+    RateLimitMiddleware, CsrfMiddleware, CorrelationIdMiddleware
+(last registered = outermost in Starlette's reverse application).
 """
 
 from __future__ import annotations