docs: add OpenAPI responses={} to all router endpoints

Add explicit HTTP status code documentation to every endpoint
across 15 router files. Each endpoint now declares all possible
response codes (200/201/204/400/401/404/409/429/502/503) with
descriptions so frontend can distinguish error types.

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

backend/app/routers/health.py

@@ -28,7 +28,15 @@ router: APIRouter = APIRouter(prefix="/api/v1/health", tags=["Health"])
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
 
-@router.get("", summary="Application health check", response_model=HealthResponse)
+@router.get(
+    "",
+    summary="Application health check",
+    response_model=HealthResponse,
+    responses={
+        200: {"description": "All components healthy"},
+        503: {"description": "fail2ban offline or component degraded"},
+    },
+)
 async def health_check(
     app_state: AppStateDep,
     server_status: ServerStatusDep,