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

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

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

app/backend/internal/mcpserver/server.go

@@ -99,7 +99,7 @@ func init() {
 	register(spec{"containr_databases_list", "List managed databases", "GET", "/databases", nil, nil, false})
 	register(spec{"containr_databases_get", "Get a database (connection URL included)", "GET", "/databases/{id}", []string{"id"}, nil, false})
 	register(spec{"containr_databases_create", "Provision a managed database. Args: name, type (postgres|mysql|mariadb|mongodb|redis|dragonfly|clickhouse), public_port (bind on all interfaces, default localhost-only).", "POST", "/databases", nil, nil, false})
-	register(spec{"containr_databases_update", "Update a database. Args: id, name, backup_schedule (empty clears), public_port — toggling recreates the container so the new bind scope applies.", "PUT", "/databases/{id}", []string{"id"}, nil, false})
+	register(spec{"containr_databases_update", "Update a database. Args: id, name, backup_schedule (empty clears), backup_target_id (empty clears), public_port — toggling recreates the container so the new bind scope applies.", "PUT", "/databases/{id}", []string{"id"}, nil, false})
 	register(spec{"containr_databases_register_external", "Register a database hosted elsewhere — probed first, password stored encrypted. Args: name (required), type (required), host (required), port, database, username, password, ssl.", "POST", "/databases/register-external", nil, []string{"name", "type", "host"}, false})
 	register(spec{"containr_databases_test_connection", "Probe a DB connection ad-hoc. Args: type, host (required), port, database, username, password, ssl.", "POST", "/databases/test-connection", nil, []string{"type", "host"}, false})
 	register(spec{"containr_databases_test_connection_by_id", "Re-probe a registered external database with its stored credentials. Args: id.", "POST", "/databases/{id}/test-connection", []string{"id"}, nil, false})
@@ -108,6 +108,13 @@ func init() {
 	register(spec{"containr_databases_restore", "Restore a database from a backup — overwrites current data. Requires confirm:true.", "POST", "/databases/{id}/restore", []string{"id"}, []string{"backup_id"}, true})
 	register(spec{"containr_databases_delete", "Delete a database and its data. Requires confirm:true.", "DELETE", "/databases/{id}", []string{"id"}, nil, true})
 
+	// S3-compatible backup targets
+	register(spec{"containr_backup_targets_list", "List S3-compatible backup targets (credentials never returned)", "GET", "/backup-targets", nil, nil, false})
+	register(spec{"containr_backup_targets_create", "Register a backup target — probed first, keys stored encrypted. Args: name, endpoint (required), bucket (required), region, prefix, access_key, secret_key, use_tls.", "POST", "/backup-targets", nil, []string{"endpoint", "bucket"}, false})
+	register(spec{"containr_backup_targets_update", "Update a backup target; empty credentials keep stored keys. Args: id, name, endpoint, bucket, region, prefix, access_key, secret_key, use_tls.", "PUT", "/backup-targets/{id}", []string{"id"}, nil, false})
+	register(spec{"containr_backup_targets_delete", "Delete a backup target — fails while databases still point at it. Requires confirm:true.", "DELETE", "/backup-targets/{id}", []string{"id"}, nil, true})
+	register(spec{"containr_backup_targets_test", "Re-probe a backup target's bucket access. Args: id.", "POST", "/backup-targets/{id}/test", []string{"id"}, nil, false})
+
 	// Cron
 	register(spec{"containr_cron_list", "List cron jobs", "GET", "/cron-jobs", nil, nil, false})
 	register(spec{"containr_cron_create", "Create a cron job. Args: service_id, schedule (5-field), command, name, timezone.", "POST", "/cron-jobs", nil, nil, false})