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/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 ---