Fix health check endpoint to return 503 when fail2ban is offline

The health check endpoint now properly indicates service unavailability:
- Returns HTTP 200 when fail2ban is online
- Returns HTTP 503 when fail2ban is offline

This allows Docker and other orchestration tools to correctly detect when
fail2ban is unreachable and automatically restart the backend container,
preventing the situation where Docker treats the container as healthy
despite fail2ban being down.

Changes:
- Update GET /api/health to return 503 on fail2ban offline
- Return appropriate JSON response bodies for each state
- Update tests to verify both online (200) and offline (503) scenarios
- Update Dockerfile HEALTHCHECK documentation
- Add Health Checks section to Deployment.md documentation

All tests pass with 100% coverage on health.py.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/routers/health.py

@@ -16,19 +16,28 @@ router: APIRouter = APIRouter(prefix="/api", tags=["Health"])
 
 @router.get("/health", summary="Application health check")
 async def health_check(server_status: ServerStatusDep) -> JSONResponse:
-    """Return 200 with application and fail2ban status.
+    """Return application and fail2ban status.
 
-    HTTP 200 is always returned so Docker health checks do not restart the
-    backend container when fail2ban is temporarily offline.  The
-    ``fail2ban`` field in the body indicates the daemon's current state.
+    Returns HTTP 200 if fail2ban is online, HTTP 503 if offline.
+    Docker health checks interpret 503 as unhealthy and restart the container
+    if fail2ban remains unreachable, ensuring the backend only runs when
+    fail2ban is available.
 
     Args:
         server_status: Injected cached server status snapshot.
 
     Returns:
-        A JSON object with ``{"status": "ok", "fail2ban": "online"|"offline"}``.
+        HTTP 200 with ``{"status": "ok", "fail2ban": "online"}`` if healthy,
+        or HTTP 503 with ``{"status": "unavailable", "fail2ban": "offline"}``
+        if fail2ban is unreachable.
     """
-    return JSONResponse(content={
-        "status": "ok",
-        "fail2ban": "online" if server_status.online else "offline",
-    })
+    if not server_status.online:
+        return JSONResponse(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            content={"status": "unavailable", "fail2ban": "offline"},
+        )
+
+    return JSONResponse(
+        status_code=status.HTTP_200_OK,
+        content={"status": "ok", "fail2ban": "online"},
+    )