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.

.env.example

@@ -29,6 +29,9 @@ ALERTMANAGER_URL=http://alertmanager:9093
 ALERTMANAGER_WEBHOOK_URL=
 GRAFANA_URL=http://grafana:3000
 PROMETHEUS_URL=http://prometheus:9090
+# Required: master key for encrypting service secrets (API keys/tokens) at rest.
+# Generate one with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
+MANAGE_ENCRYPTION_KEY=replace-with-a-fernet-key
 BACKEND_CACHE_DIR=./backend-cache
 
 # Auth