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.

docker-compose.yml

@@ -30,6 +30,7 @@ services:
       ALERTMANAGER_WEBHOOK_URL: ${ALERTMANAGER_WEBHOOK_URL:-}
       GRAFANA_URL: ${GRAFANA_URL:-http://grafana:3000}
       PROMETHEUS_URL: ${PROMETHEUS_URL:-http://prometheus:9090}
+      MANAGE_ENCRYPTION_KEY: ${MANAGE_ENCRYPTION_KEY:?generate one with python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"}
     volumes:
       - ${BACKEND_CACHE_DIR:-./backend-cache}:/app/backend/.cache
     restart: unless-stopped