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>

app/backend/sqlc/queries/backup_targets.sql

@@ -0,0 +1,43 @@
+-- name: ListBackupTargetsByUser :many
+SELECT backup_targets.*
+FROM backup_targets
+WHERE user_id = sqlc.arg(user_id)
+ORDER BY created_at DESC;
+
+-- name: GetBackupTargetByIDAndUser :one
+SELECT backup_targets.*
+FROM backup_targets
+WHERE id = sqlc.arg(id) AND user_id = sqlc.arg(user_id);
+
+-- name: GetBackupTargetByID :one
+SELECT backup_targets.*
+FROM backup_targets
+WHERE id = sqlc.arg(id);
+
+-- name: CreateBackupTarget :exec
+INSERT INTO backup_targets (id, user_id, name, endpoint, bucket, region, prefix, access_key, secret_key, use_tls, created_at, updated_at)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12);
+
+-- name: UpdateBackupTargetByIDAndUser :exec
+UPDATE backup_targets
+SET name = $1, endpoint = $2, bucket = $3, region = $4, prefix = $5, access_key = $6, secret_key = $7, use_tls = $8, updated_at = $9
+WHERE id = $10 AND user_id = $11;
+
+-- name: DeleteBackupTargetByIDAndUser :exec
+DELETE FROM backup_targets
+WHERE id = $1 AND user_id = $2;
+
+-- name: SetDatabaseBackupTargetByIDAndUser :exec
+UPDATE database_services
+SET backup_target_id = $1, updated_at = $2
+WHERE id = $3 AND user_id = $4;
+
+-- name: SetDatabaseBackupRemoteKeyByID :exec
+UPDATE database_backups
+SET remote_key = $1
+WHERE id = $2;
+
+-- name: CountDatabasesUsingBackupTarget :one
+SELECT COUNT(*)
+FROM database_services
+WHERE backup_target_id = sqlc.arg(target_id);