feat(service-credential-tester): slice 1 — backend test endpoint + per-type routines

TestResult dataclass + translate_connection_error shared helper in base.py.
test_callable field on ServiceDefinition (default None). 7 per-type
test_connection routines (qbittorrent, prometheus via Grafana gateway,
alertmanager, jellyfin, authentik, ssh_tasks via build_ssh_client, nextcloud).
POST /api/services/test endpoint: validation-first (422 on malformed config),
dispatch, no-persistence, no-secret-logs. backups has test_callable=None.
qBit 'Fails.' → specific auth message (resolves #3 at API layer).
Backend: 362 pytest pass (+31 new), ruff clean. Frontend: build green.

backend/src/media_library_viewer_api/integrations/prometheus.py

@@ -2,15 +2,71 @@
 
 from __future__ import annotations
 
+from typing import TYPE_CHECKING, Any
+
+import requests
+
 from media_library_viewer_api.integrations.base import (
     SecretField,
     ServiceBaseUrl,
     ServiceConfigBase,
     ServiceDefinition,
+    TestResult,
     WidgetConfigBase,
+    translate_connection_error,
     widget_kind,
 )
 
+if TYPE_CHECKING:
+    from media_library_viewer_api.services.settings_store import SettingsStore
+
+
+def test_connection(
+    config: dict[str, Any],
+    secrets: dict[str, str],
+    store: SettingsStore,
+) -> TestResult:
+    """POST {grafana_url}/api/ds/query with expr 'up' via the Grafana gateway."""
+    try:
+        grafana_url = str(config.get("grafana_url") or "").rstrip("/")
+        api_key = str(secrets.get("grafana_api_key") or "")
+        datasource_uid = str(config.get("datasource_uid") or "prometheus")
+        timeout = int(config.get("timeout_seconds") or 10)
+        if not grafana_url:
+            return TestResult(ok=False, detail="Grafana gateway URL is required.")
+        if not api_key:
+            return TestResult(ok=False, detail="Grafana API key is required.")
+        body = {
+            "queries": [
+                {
+                    "datasource": {"uid": datasource_uid, "type": "prometheus"},
+                    "expr": "up",
+                    "format": "time_series",
+                    "intervalMs": 15000,
+                    "maxDataPoints": 1,
+                    "refId": "A",
+                }
+            ],
+            "from": "now-1m",
+            "to": "now",
+        }
+        resp = requests.post(
+            f"{grafana_url}/api/ds/query",
+            json=body,
+            timeout=timeout,
+            headers={"Authorization": f"Bearer {api_key}", "Content-Type": "application/json"},
+        )
+        resp.raise_for_status()
+        return TestResult(
+            ok=True,
+            detail="Grafana gateway reachable.",
+            evidence="Gateway reachable; datasource responded.",
+        )
+    except requests.HTTPError as exc:
+        return translate_connection_error(exc, context="Prometheus via Grafana")
+    except Exception as exc:
+        return translate_connection_error(exc, context="Prometheus via Grafana")
+
 
 class PrometheusConfig(ServiceConfigBase):
     """Non-secret Prometheus-via-Grafana gateway config."""
@@ -99,4 +155,5 @@ DEFINITION = ServiceDefinition(
             refresh_interval_ms=60_000,
         ),
     ],
+    test_callable=test_connection,
 )