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/server.py

@@ -30,6 +30,11 @@ router: APIRouter = APIRouter(prefix="/api/v1/server", tags=["Server"])
     "/settings",
     response_model=ServerSettingsResponse,
     summary="Return fail2ban server-level settings",
+    responses={
+        200: {"description": "Server settings returned", "model": ServerSettingsResponse},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def get_server_settings(
     request: Request,
@@ -59,6 +64,12 @@ async def get_server_settings(
     "/settings",
     status_code=status.HTTP_204_NO_CONTENT,
     summary="Update fail2ban server-level settings",
+    responses={
+        204: {"description": "Settings updated successfully"},
+        400: {"description": "Set command rejected by fail2ban"},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def update_server_settings(
     request: Request,
@@ -88,6 +99,12 @@ async def update_server_settings(
     status_code=status.HTTP_200_OK,
     summary="Flush and re-open fail2ban log files",
     response_model=FlushLogsResponse,
+    responses={
+        200: {"description": "Logs flushed successfully", "model": FlushLogsResponse},
+        400: {"description": "Command rejected by fail2ban"},
+        401: {"description": "Session missing, expired, or invalid"},
+        502: {"description": "fail2ban unreachable"},
+    },
 )
 async def flush_logs(
     request: Request,