fix(api): return 503 instead of 500 when Jellyfin/SSH not configured

On a fresh deploy with no Jellyfin service configured yet,
get_jellyfin_client (and get_user_id / get_ssh_client) raised a plain
RuntimeError, which bubbled up as a 500 traceback on every
Jellyfin-dependent route (dashboard counts/libraries/activity, media,
users). Convert those RuntimeErrors to HTTPException(503) with a clear
detail message so FastAPI returns a clean 503 JSON response instead of
a 500, and the frontend can render a not-configured state.

- dependencies.py: get_jellyfin_client (no service / missing creds),
  get_user_id (no users discovered), and get_ssh_client (no SSH machine
  + no legacy key path) now raise HTTPException(503, detail=...).
- tests/test_api.py: added
  TestDashboard.test_jellyfin_endpoints_return_503_when_not_configured
  covering /api/dashboard/counts and /activity.

ruff clean; 240 backend tests pass.

backend/src/media_library_viewer_api/dependencies.py

@@ -163,11 +163,17 @@ def get_jellyfin_client(request: Request = None) -> JellyfinClient:
     service_id = _request_jellyfin_service_id(request)
     service = _service_record(store, "jellyfin", service_id)
     if service is None:
-        raise RuntimeError("No Jellyfin service is configured. Add a Jellyfin service on the Services page.")
+        raise HTTPException(
+            status_code=503,
+            detail="No Jellyfin service is configured. Add a Jellyfin service on the Services page.",
+        )
     base_url = str(service.get("config", {}).get("base_url") or "")
     api_key = str(service.get("secrets", {}).get("api_key") or "")
     if not base_url or not api_key:
-        raise RuntimeError("Jellyfin service is missing base_url or api_key. Edit it on the Services page.")
+        raise HTTPException(
+            status_code=503,
+            detail="Jellyfin service is missing base_url or api_key. Edit it on the Services page.",
+        )
     cache_key = (service["id"], base_url, api_key)
     return _jellyfin_client_for(cache_key)
 
@@ -244,7 +250,10 @@ def get_ssh_client(request: Request = None):
         "set" if settings.ssh_password else "missing",
     )
     if not settings.ssh_key_path:
-        raise RuntimeError("No SSH machine is configured and SSH key settings must be configured")
+        raise HTTPException(
+            status_code=503,
+            detail="No SSH machine is configured and SSH key settings must be configured",
+        )
     return _ssh_client_for(
         (
             "legacy",
@@ -280,5 +289,8 @@ def get_user_id(request: Request = None) -> str:
     client = get_jellyfin_client(request)
     users = client.users()
     if not users:
-        raise RuntimeError("No Jellyfin users found and no user_id configured on the service")
+        raise HTTPException(
+            status_code=503,
+            detail="No Jellyfin users found and no user_id configured on the service",
+        )
     return users[0]["Id"]

backend/tests/test_api.py

@@ -211,6 +211,22 @@ class TestDashboard:
         data = response.json()
         assert len(data) == 2
 
+    def test_jellyfin_endpoints_return_503_when_not_configured(self, test_client):
+        # Remove the mocked Jellyfin dependency so the real one runs; with no
+        # Jellyfin service seeded, endpoints must degrade to 503, not 500.
+        app.dependency_overrides.pop(get_jellyfin_client, None)
+        app.dependency_overrides.pop(get_user_id, None)
+        try:
+            for path in ("/api/dashboard/counts", "/api/dashboard/activity"):
+                response = test_client.get(path)
+                assert response.status_code == 503, path
+                detail = response.json()["detail"]
+                assert "configured" in detail, path
+        finally:
+            # Restore the mocks for subsequent tests in this fixture session.
+            app.dependency_overrides[get_jellyfin_client] = lambda: MagicMock()
+            app.dependency_overrides[get_user_id] = lambda: "user123"
+
 
 # --- Settings reset ---