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/base.py

@@ -15,11 +15,16 @@ map. There is no runtime plugin loading.
 
 from __future__ import annotations
 
+import asyncio
 from dataclasses import dataclass, field
-from typing import Annotated, Any
+from typing import TYPE_CHECKING, Annotated, Any, Callable
 
+import requests
 from pydantic import BaseModel, BeforeValidator, Field
 
+if TYPE_CHECKING:
+    from media_library_viewer_api.services.settings_store import SettingsStore
+
 
 def _validate_service_base_url(value: Any) -> str:
     """Require an absolute http(s) URL for service ``base_url`` fields.
@@ -93,6 +98,20 @@ class WidgetKind:
     config_model: type[WidgetConfigBase] | None = None
 
 
+@dataclass(frozen=True)
+class TestResult:
+    """Outcome of a credential/connectivity test for a service instance."""
+
+    ok: bool
+    detail: str
+    evidence: str | None = None
+
+
+#: A test routine receives (config, secrets, store). The store is needed for
+#: ssh_tasks (SSH-key resolution). Other types ignore it.
+TestCallable = Callable[[dict[str, Any], dict[str, str], "SettingsStore"], TestResult]
+
+
 @dataclass(frozen=True)
 class ServiceDefinition:
     """Closed description of an external service type."""
@@ -103,6 +122,7 @@ class ServiceDefinition:
     config_model: type[ServiceConfigBase]
     secret_fields: list[SecretField]
     widget_kinds: list[WidgetKind]
+    test_callable: TestCallable | None = None
 
     @property
     def config_schema(self) -> dict[str, Any]:
@@ -148,3 +168,58 @@ def validate_config(model_cls: type[BaseModel], config: dict[str, Any] | None) -
     """Validate a config dict against a Pydantic model and return the cleaned dict."""
     instance = model_cls.model_validate(config or {})
     return instance.model_dump(exclude_none=True)
+
+
+def translate_connection_error(exc: Exception, *, context: str = "") -> TestResult:
+    """Map a common connection/auth exception to a human-friendly TestResult.
+
+    Handles patterns extracted from ``test_machine_ssh`` (settings.py) plus
+    HTTP-client patterns from the widget sources. Each per-type test routine
+    calls this for unexpected exceptions, but handles its **type-specific**
+    auth failures directly (e.g., qBit ``"Fails."``).
+    """
+    message = str(exc)
+    lowered = message.lower()
+
+    # Auth failures (HTTP 401/403)
+    if isinstance(exc, requests.HTTPError):
+        status_code = exc.response.status_code if exc.response is not None else 0
+        if status_code in (401, 403):
+            return TestResult(
+                ok=False,
+                detail=f"Authentication failed — the service rejected the credentials ({status_code}).",
+            )
+    if "authentication failed" in lowered or "no authentication methods available" in lowered:
+        return TestResult(ok=False, detail="Authentication failed — check the credentials, API key, or SSH key.")
+
+    # Timeout (before OSError check, since requests.Timeout is a subclass of OSError)
+    if isinstance(exc, (requests.Timeout, TimeoutError, asyncio.TimeoutError)):
+        return TestResult(ok=False, detail="Connection timed out — the service did not respond in time.")
+
+    # Connection refused / DNS / unreachable
+    if isinstance(exc, (requests.ConnectionError, ConnectionRefusedError, OSError)):
+        if (
+            "name or service not known" in lowered
+            or "nodename nor servname" in lowered
+            or "getaddrinfo failed" in lowered
+        ):
+            return TestResult(ok=False, detail="Host not found — check the URL/hostname for typos.")
+        return TestResult(
+            ok=False,
+            detail="Connection refused — the service is not reachable at the configured address.",
+        )
+
+    # SSL / certificate errors
+    if "ssl" in lowered or "certificate" in lowered:
+        return TestResult(ok=False, detail="SSL/TLS error — the service's certificate is invalid or untrusted.")
+
+    # SSH banner (from test_machine_ssh pattern)
+    if "protocol banner" in lowered:
+        return TestResult(
+            ok=False,
+            detail="SSH banner not received — confirm the SSH service is running and the port is correct.",
+        )
+
+    # Fallback
+    prefix = f"{context}: " if context else ""
+    return TestResult(ok=False, detail=f"{prefix}{message[:200]}")