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/ssh_tasks.py

@@ -8,14 +8,74 @@ and shown as history on the instance's service page.
 
 from __future__ import annotations
 
+from typing import TYPE_CHECKING, Any
+
 from media_library_viewer_api.integrations.base import (
     SecretField,
     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:
+    """Build an SSH client via build_ssh_client and attempt .connect().
+
+    Reuses the same error-translation patterns as test_machine_ssh (banner,
+    auth failed). Known-host recording is preserved.
+    """
+    from media_library_viewer_api.services.task_runner import build_ssh_client
+    from media_library_viewer_api.widgets.sources import ServiceRecord
+
+    host = str(config.get("host") or "").strip()
+    port = int(config.get("port") or 22)
+    try:
+        service = ServiceRecord(
+            id="",
+            service_type="ssh_tasks",
+            name="test",
+            config=config,
+            secrets=secrets,
+            enabled=True,
+        )
+        client = build_ssh_client(store, service)
+        try:
+            client.connect()
+        except Exception as exc:
+            lowered = str(exc).lower()
+            if "protocol banner" in lowered:
+                return TestResult(
+                    ok=False,
+                    detail=f"SSH banner not received from {host}:{port}; confirm the SSH service is running.",
+                )
+            if "no authentication methods available" in lowered or "authentication failed" in lowered:
+                return TestResult(
+                    ok=False,
+                    detail=f"SSH authentication failed for {host}:{port}; check the SSH key, passphrase, or username.",
+                )
+            return translate_connection_error(exc, context=f"SSH {host}:{port}")
+        finally:
+            client.close()
+        return TestResult(
+            ok=True,
+            detail=f"SSH connection succeeded for {host}:{port}.",
+            evidence=f"Connected to {host}:{port}",
+        )
+    except ValueError as exc:
+        return TestResult(ok=False, detail=str(exc))
+    except Exception as exc:
+        return translate_connection_error(exc, context=f"SSH {host}:{port}")
+
 
 class SshTasksConfig(ServiceConfigBase):
     """Non-secret SSH task runner config.
@@ -57,4 +117,5 @@ DEFINITION = ServiceDefinition(
             refresh_interval_ms=0,
         ),
     ],
+    test_callable=test_connection,
 )