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/setup.py

@@ -23,9 +23,12 @@ router = APIRouter(prefix="/api/v1/setup", tags=["setup"])
 
 
 @router.get(
-    "",
+    "/",
     response_model=SetupStatusResponse,
     summary="Check whether setup has been completed",
+    responses={
+        200: {"description": "Setup status returned", "model": SetupStatusResponse},
+    },
 )
 async def get_setup_status(app: AppDep) -> SetupStatusResponse:
     """Return whether the initial setup wizard has been completed.
@@ -43,6 +46,11 @@ async def get_setup_status(app: AppDep) -> SetupStatusResponse:
     response_model=SetupResponse,
     status_code=status.HTTP_201_CREATED,
     summary="Run the initial setup wizard",
+    responses={
+        201: {"description": "Setup completed successfully", "model": SetupResponse},
+        400: {"description": "Validation error in request body"},
+        409: {"description": "Setup already completed"},
+    },
 )
 async def post_setup(
     app: AppDep,
@@ -88,6 +96,9 @@ async def post_setup(
     "/timezone",
     response_model=SetupTimezoneResponse,
     summary="Return the configured IANA timezone",
+    responses={
+        200: {"description": "Timezone returned", "model": SetupTimezoneResponse},
+    },
 )
 async def get_timezone(settings: SettingsDep) -> SetupTimezoneResponse:
     """Return the IANA timezone configured during the initial setup wizard.