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/qbittorrent.py

@@ -7,15 +7,47 @@ password), and three widget kinds (totals, active, speed). Models on
 
 from __future__ import annotations
 
+from typing import TYPE_CHECKING, Any
+
+from media_library_viewer_api.clients.qbittorrent import QbittorrentClient
 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:
+    """Login + probe maindata; surface auth failures specifically."""
+    try:
+        base_url = str(config.get("base_url") or "")
+        username = str(secrets.get("username") or "")
+        password = str(secrets.get("password") or "")
+        timeout = int(config.get("timeout_seconds") or 10)
+        client = QbittorrentClient(base_url, username, password, timeout=timeout)
+        data = client.maindata()
+        version = str(data.get("server_state", {}).get("qbittorrent_version", "") or "connected")
+        return TestResult(ok=True, detail="Connected to qBittorrent.", evidence=version)
+    except RuntimeError as exc:
+        lowered = str(exc).lower()
+        if "login failed" in lowered:
+            return TestResult(ok=False, detail="Authentication failed — qBittorrent rejected the credentials.")
+        return translate_connection_error(exc, context="qBittorrent")
+    except Exception as exc:
+        return translate_connection_error(exc, context="qBittorrent")
+
 
 class QbittorrentConfig(ServiceConfigBase):
     """Non-secret qBittorrent connection config."""
@@ -65,4 +97,5 @@ DEFINITION = ServiceDefinition(
             refresh_interval_ms=5_000,
         ),
     ],
+    test_callable=test_connection,
 )