feat(services): backend service registry foundation (encryption, definitions, CRUD)

PR 1 of 4 for the runtime service registry change.

- Add Fernet encryption helper (services/secrets.py) with a required
  MANAGE_ENCRYPTION_KEY; validate it on startup.
- Add closed integrations/ registry with Pydantic config + widget-config
  definitions for grafana, prometheus, jellyfin, nextcloud, and ssh_tasks.
- Add services + service_task_runs tables and SettingsStore CRUD with
  cascade-delete (defensive until widgets carry service_id).
- Add /api/services/types and /api/services/instances CRUD (encrypted secrets,
  secrets_set flags only; never plaintext).
- Declare cryptography as a direct dependency.
- Require MANAGE_ENCRYPTION_KEY in compose + .env.example + README.
- Add 25 backend tests (registry, encryption, CRUD, cascade, task-run history).

Verification: ruff clean; pytest 225 passed; frontend lint/build green.

backend/src/media_library_viewer_api/integrations/ssh_tasks.py

@@ -0,0 +1,60 @@
+"""SSH task runner service definition.
+
+An ``ssh_tasks`` instance is an SSH endpoint that can run reusable saved tasks.
+Tasks themselves stay in the global saved-task registry; the instance only owns
+transport (host/port/user/key). Every run is recorded in ``service_task_runs``
+and shown as history on the instance's service page.
+"""
+
+from __future__ import annotations
+
+from media_library_viewer_api.integrations.base import (
+    SecretField,
+    ServiceConfigBase,
+    ServiceDefinition,
+    WidgetConfigBase,
+    widget_kind,
+)
+
+
+class SshTasksConfig(ServiceConfigBase):
+    """Non-secret SSH task runner config.
+
+    The SSH key itself lives in the saved SSH-key registry and is referenced by
+    ``ssh_key_id``. An optional ``passphrase`` is stored as a secret.
+    """
+
+    host: str
+    port: int = 22
+    username: str = ""
+    ssh_key_id: str = ""
+    timeout_seconds: int = 30
+
+
+class SshTaskOutputWidgetConfig(WidgetConfigBase):
+    """Output of a saved task run on this instance."""
+
+    task_id: str
+    # service_id is implicit (the widget's service); allow overriding per-widget.
+    service_id: str | None = None
+
+
+DEFINITION = ServiceDefinition(
+    service_type="ssh_tasks",
+    name="SSH task runner",
+    description="Run reusable saved tasks over SSH and keep run history.",
+    config_model=SshTasksConfig,
+    secret_fields=[
+        SecretField(key="passphrase", label="Key passphrase", helper="Optional"),
+    ],
+    widget_kinds=[
+        widget_kind(
+            kind="task_output",
+            name="Task output",
+            description="Output of a saved task run.",
+            model_cls=SshTaskOutputWidgetConfig,
+            default_config={"task_id": ""},
+            refresh_interval_ms=0,
+        ),
+    ],
+)