feat(databases): S3-compatible backup targets — offsite archives + remote restore

Phase D3. Managed databases can ship backup archives to a configured
S3-compatible destination (AWS S3, MinIO, Garage, …) while the local
copy remains intact.

- backup_targets table: owner-scoped endpoint/bucket/region/prefix +
  encrypted access/secret keys (enc:v1:, never returned, has_credentials)
- create/update probe bucket access before storing; empty keys on update
  keep stored credentials; delete refused (409) while referenced
- databases.backup_target_id assigns a target; external DBs rejected
- backup pipeline uploads via a holder container over docker cp streams;
  remote failure keeps the local archive (logged), row gets remote:true
- restore prefers the local archive; on a missing local copy it downloads
  the remote object into the backup volume first
- holder containers must be started before archive ops — docker cp does
  not resolve volume mounts on never-started (Created) containers, and
  removing the source mid-stream stalls the reader without EOF
- CLI: backup-targets list|add|update|remove|test + databases update
  --backup-target; MCP: 5 new tools; Settings UI card + per-database
  target picker + offsite badge on backups; OpenAPI + llms.txt + CLI.md

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

llms.txt

@@ -23,6 +23,7 @@ Base URL: `https://<host>/api/v1`. OpenAPI: `docs/api/openapi.yaml`.
 - **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.
 - **Public DB port**: managed databases bind `127.0.0.1` by default; `public_port:true` on create or `PUT /databases/{id} {public_port}` binds `0.0.0.0`. Toggling recreates the container — the data volume persists.
+- **Backup targets**: S3-compatible destinations (`/backup-targets` CRUD + `POST /backup-targets/{id}/test`). Endpoints/buckets are probed before storing; keys encrypted at rest (`has_credentials` only). Assign via `PUT /databases/{id} {backup_target_id}` — empty string clears; external databases reject assignment. Managed backups stay local; a copy ships to `[prefix/]databases/<db-id>/<archive>` and rows expose `remote:true`. Restore prefers the local archive and falls back to the remote object. Deleting a target is refused (409) while databases reference it.
 - **Identifiers**: all ids are UUIDs. Tokens: `cnp_` user PATs, `cagt_` agent onboarding tokens.
 
 ## Core objects