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/main.py

@@ -23,6 +23,7 @@ from media_library_viewer_api.observability import (
 )
 from media_library_viewer_api.routers import backups as backups_router
 from media_library_viewer_api.routers import dashboard, files, jobs, media, monitoring, tasks, users
+from media_library_viewer_api.routers import services as services_router
 from media_library_viewer_api.routers import widgets as widgets_router
 from media_library_viewer_api.routers.settings import router as settings_router
 
@@ -38,6 +39,9 @@ async def lifespan(app: FastAPI):
     settings = get_settings()
     configure_logging(settings.log_level, settings.log_format)
     validate_auth_settings(settings)
+    from media_library_viewer_api.services.secrets import validate_encryption_key
+
+    validate_encryption_key()
     logger.info("Backend startup complete: %s", describe_settings(settings))
     logger.info("Managed known_hosts will be populated lazily on first successful SSH connection")
     try:
@@ -143,6 +147,7 @@ app.include_router(tasks.router)
 app.include_router(settings_router)
 app.include_router(backups_router.router)
 app.include_router(widgets_router.router)
+app.include_router(services_router.router)
 
 
 @app.get("/api/health")