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/api/routes.go

@@ -354,6 +354,13 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			// One-off exec console (docker exec, 30s ceiling)
 			authed.POST("/services/:id/exec", handleExecInService)
 
+			// S3-compatible backup targets — owner-scoped, keys encrypted at rest.
+			authed.GET("/backup-targets", databaseHandler.ListBackupTargets)
+			authed.POST("/backup-targets", databaseHandler.CreateBackupTarget)
+			authed.PUT("/backup-targets/:id", databaseHandler.UpdateBackupTarget)
+			authed.DELETE("/backup-targets/:id", databaseHandler.DeleteBackupTarget)
+			authed.POST("/backup-targets/:id/test", databaseHandler.TestBackupTarget)
+
 			// Private-image registries — owner-scoped, credentials encrypted at rest.
 			authed.GET("/registries", handleListRegistries)
 			authed.POST("/registries", handleCreateRegistry)