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/databases.sql

@@ -78,7 +78,7 @@ SET status = $1, connection_url = $2, updated_at = $3
 WHERE id = $4;
 
 -- name: ListDatabaseBackupsByDatabaseAndUser :many
-SELECT b.id, b.database_id, b.size, b.status, b.backup_path, b.created_at, b.completed_at
+SELECT b.id, b.database_id, b.size, b.status, b.backup_path, b.remote_key, b.created_at, b.completed_at
 FROM database_backups b
 JOIN database_services s ON s.id = b.database_id
 WHERE b.database_id = $1 AND s.user_id = $2
@@ -86,7 +86,7 @@ ORDER BY b.created_at DESC
 LIMIT $3;
 
 -- name: GetDatabaseBackupByIDAndDatabaseAndUser :one
-SELECT b.id, b.database_id, b.size, b.status, b.backup_path, b.created_at, b.completed_at
+SELECT b.id, b.database_id, b.size, b.status, b.backup_path, b.remote_key, b.created_at, b.completed_at
 FROM database_backups b
 JOIN database_services s ON s.id = b.database_id
 WHERE b.id = $1 AND b.database_id = $2 AND s.user_id = $3;