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/nextcloud.py

@@ -0,0 +1,32 @@
+"""Nextcloud service definition.
+
+Nextcloud is included as a proof-of-concept third-party service. It has no
+dashboard widgets yet; its service page holds connection config only.
+"""
+
+from __future__ import annotations
+
+from media_library_viewer_api.integrations.base import (
+    SecretField,
+    ServiceConfigBase,
+    ServiceDefinition,
+)
+
+
+class NextcloudConfig(ServiceConfigBase):
+    """Non-secret Nextcloud connection config."""
+
+    base_url: str
+    username: str = ""
+
+
+DEFINITION = ServiceDefinition(
+    service_type="nextcloud",
+    name="Nextcloud",
+    description="Self-hosted files and collaboration.",
+    config_model=NextcloudConfig,
+    secret_fields=[
+        SecretField(key="app_password", label="App password", required=True),
+    ],
+    widget_kinds=[],
+)