feat(databases): backup import — upload .tar.gz as a restore point

D5. POST /databases/:id/backups/import accepts a gzipped tar archive
(multipart `file` field or raw application/gzip body, 2 GiB ceiling),
stages it to a temp file, streams it into the backup volume via the
started holder container, and registers a completed backup row — a
first-class restore point that also ships offsite when a target is
assigned. External databases rejected.

Also fixes `containr databases restore` — it never sent database_id,
which the API requires, so every CLI restore failed validation.

CLI: databases import-backup <id> <file|-> (stdin support via new
client.DoRaw). UI: Import button in the backups row. OpenAPI + docs.

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

@@ -398,6 +398,7 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			authed.POST("/databases/:id/action", databaseHandler.PerformDatabaseAction)
 			authed.POST("/databases/:id/backup", databaseHandler.CreateBackup)
 			authed.POST("/databases/:id/restore", databaseHandler.RestoreBackup)
+			authed.POST("/databases/:id/backups/import", databaseHandler.ImportBackup)
 			authed.GET("/databases/:id/backups/:bid/download", databaseHandler.DownloadBackup)
 
 			authed.POST("/projects/:id/preview-environments", handleCreatePreviewEnvironment)