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/docker/client.go

@@ -238,6 +238,11 @@ func (c *Client) CopyFromContainer(ctx context.Context, containerID, srcPath str
 	return reader, err
 }
 
+// CopyToContainer copies a tar archive stream into a container path.
+func (c *Client) CopyToContainer(ctx context.Context, containerID, dstPath string, content io.Reader) error {
+	return c.cli.CopyToContainer(ctx, containerID, dstPath, content, container.CopyToContainerOptions{})
+}
+
 // GetContainerStats returns real-time resource usage statistics for a container
 func (c *Client) GetContainerStats(ctx context.Context, containerID string, stream bool) (*container.StatsResponseReader, error) {
 	resp, err := c.cli.ContainerStats(ctx, containerID, stream)