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>

docs/guides/CLI.md

@@ -47,7 +47,9 @@ deployments list|get|logs|rollback|cancel
 logs <service-id> [--follow]     runtime logs (SSE follow)
 exec <service-id> -- <cmd>       one-off container command
 variables list|set|unset         env vars (secrets masked; --redeploy applies them)
-databases list|get|create|update|delete|action|backup|restore|download-backup
+databases list|get|create|update|delete|action|backup|restore|download-backup|import-backup
+                               import-backup <id> <file.tar.gz|-> uploads an archive as a restore
+                               point; ships offsite too when a backup target is assigned
                                create --public exposes the port on all interfaces;
                                update <id> --public/--public=false toggles it (container recreates, data persists)
 databases register <name>      register an external database (--type --host --port --database --username --password --ssl; probed before storing)