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/models/response.py

@@ -480,3 +480,53 @@ class FlushLogsResponse(BanGuiBaseModel):
     """
 
     message: str = Field(..., description="Human-readable result message from fail2ban.")
+
+
+class ReadyCheck(BanGuiBaseModel):
+    """Result of a single readiness subsystem check.
+
+    Fields:
+        name: Subsystem name (e.g., "database", "fail2ban", "config_dir").
+        healthy: True when the subsystem is reachable/operational.
+        message: Optional error message describing the failure.
+    """
+
+    name: str = Field(..., description="Subsystem name.")
+    healthy: bool = Field(..., description="True when the subsystem is operational.")
+    message: str | None = Field(
+        default=None,
+        description="Error detail when the check fails.",
+    )
+
+
+class ReadyResponse(BanGuiBaseModel):
+    """Structured readiness check response for the ``/health/ready`` endpoint.
+
+    Fields:
+        status: "ok" when all checks pass, "error" when at least one failed.
+        checks: Per-subsystem result list.
+        failed_count: Number of checks that returned healthy=False.
+
+    Example:
+        ```python
+        # All healthy (HTTP 200)
+        {"status": "ok", "checks": [...], "failed_count": 0}
+
+        # Some failed (HTTP 503)
+        {"status": "error", "checks": [...], "failed_count": 2}
+        ```
+    """
+
+    status: Literal["ok", "error"] = Field(
+        ...,
+        description="'ok' when all checks pass, 'error' when at least one fails.",
+    )
+    checks: list[ReadyCheck] = Field(
+        default_factory=list,
+        description="Per-subsystem check results.",
+    )
+    failed_count: int = Field(
+        ...,
+        ge=0,
+        description="Number of checks that returned healthy=False.",
+    )