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>

docs/guides/CLI.md

@@ -52,6 +52,10 @@ databases list|get|create|update|delete|action|backup|restore|download-backup
                                update <id> --public/--public=false toggles it (container recreates, data persists)
 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
+backup-targets list|add|update|remove|test
+                               S3-compatible archive destinations; add/update probe the bucket
+                               first, empty --access-key/--secret-key on update keep stored keys;
+                               update <db-id> --backup-target assigns a target ("" clears it)
 cron list|get|create|delete|trigger|executions
 templates list|get|create|delete|deploy
 nodes list|get|delete|tokens     node agents + onboarding tokens (admin)