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>

llms.txt

@@ -21,6 +21,7 @@ Base URL: `https://<host>/api/v1`. OpenAPI: `docs/api/openapi.yaml`.
 - **Builders & resources**: service field `builder` = `auto|railpack|nixpacks|dockerfile|static` (auto detects; static builds assets with `static_build_cmd` and serves `static_dir` via nginx:80). `cpu`/`memory` are hard caps; `cpu_reserve`/`memory_reserve` are soft reservations (CPU shares / MemoryReservation) — empty string clears. `app_settings.default_cpu`/`default_memory` set instance defaults; `app_settings.capacity_policy=block` rejects deploys exceeding node memory (default `warn`).
 - **Clone & move**: `POST /services/{id}/clone` `{name?, project_id?, environment?}` copies config, volumes, domains, access gates, and variables (secret ciphertext verbatim; clone starts stopped). `POST /services/{id}/move` `{project_id}` re-parents a service — both require ownership of source and target; name conflicts → `CONFLICT`. Cloned domains are verbatim copies: the same hostname on two services makes routing ambiguous.
 - **Registries**: `GET|POST /registries`, `PUT|DELETE /registries/{id}` — owner-scoped private-image pull credentials, encrypted at rest, never returned (`has_password` signals presence). `host` matched against the image reference on image deploys (`docker.io` for Docker Hub; scheme/case normalized). Empty `password` on update keeps the stored credential.
+- **External databases**: `POST /databases/register-external` (probes first; `{name,type,host,port,database,username,password,ssl}`), `POST /databases/test-connection` (ad-hoc probe → `{ok,latency_ms,error}`), `POST /databases/{id}/test-connection` (re-probe with stored credentials). Rows carry `provider:"external"` + `external{host,port,database,username,has_password,ssl}`; passwords encrypted at rest, never returned. Lifecycle actions and backups reject external rows — no container exists.
 - **Identifiers**: all ids are UUIDs. Tokens: `cnp_` user PATs, `cagt_` agent onboarding tokens.
 
 ## Core objects