feat: external databases — register, probe, encrypted credentials

Databases hosted outside Containr can now be registered alongside
managed ones. The connection is probed before storing (protocol-level
for postgres/redis-family via real drivers, TCP dial for the rest),
credentials are encrypted at rest and never returned, and external rows
skip managed lifecycle actions, backups, and Docker metrics.

- POST /databases/register-external — probe + create
- POST /databases/test-connection — ad-hoc probe (ok/latency_ms/error)
- POST /databases/{id}/test-connection — re-probe stored credentials
- provider/external fields on database services; privileged reads get a
  composed connection URL with loopback hosts rewritten to the caller's
- CLI: databases register + test-connection; MCP: 3 new tools
- UI: managed/external toggle in the create modal, external badge and
  host display, per-row test button, backups section hidden
- Migration widens the plan CHECK to admit 'external'

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/guides/CLI.md

@@ -48,6 +48,8 @@ logs <service-id> [--follow]     runtime logs (SSE follow)
 exec <service-id> -- <cmd>       one-off container command
 variables list|set|unset         env vars (secrets masked; --redeploy applies them)
 databases list|get|create|delete|action|backup|restore|download-backup
+databases register <name>      register an external database (--type --host --port --database --username --password --ssl; probed before storing)
+databases test-connection      probe a connection — by id, or ad-hoc flags
 cron list|get|create|delete|trigger|executions
 templates list|get|create|delete|deploy
 nodes list|get|delete|tokens     node agents + onboarding tokens (admin)