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/backuptargets_test.go

@@ -0,0 +1,60 @@
+package api
+
+import (
+	"archive/tar"
+	"bytes"
+	"io"
+	"testing"
+)
+
+func TestNormalizeBackupTargetRequest(t *testing.T) {
+	req := backupTargetRequest{
+		Name:     " garage ",
+		Endpoint: "https://garage.local:3900/",
+		Bucket:   " backups ",
+		Prefix:   " /prod/dbs/ ",
+	}
+	if err := normalizeBackupTargetRequest(&req); err != nil {
+		t.Fatalf("normalize failed: %v", err)
+	}
+	if req.Endpoint != "garage.local:3900" {
+		t.Fatalf("endpoint not normalized: %q", req.Endpoint)
+	}
+	if req.Bucket != "backups" || req.Prefix != "prod/dbs" {
+		t.Fatalf("bucket/prefix not trimmed: %q %q", req.Bucket, req.Prefix)
+	}
+
+	for i, bad := range []backupTargetRequest{
+		{Endpoint: "e", Bucket: "b"}, // no name
+		{Name: "n", Bucket: "b"},     // no endpoint
+		{Name: "n", Endpoint: "e"},   // no bucket
+	} {
+		if err := normalizeBackupTargetRequest(&bad); err == nil {
+			t.Fatalf("case %d: expected error", i)
+		}
+	}
+}
+
+func TestArchiveReaderUnwrapsTar(t *testing.T) {
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	payload := []byte("backup-bytes")
+	if err := tw.WriteHeader(&tar.Header{Name: "b.tar.gz", Mode: 0o644, Size: int64(len(payload))}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := tw.Write(payload); err != nil {
+		t.Fatal(err)
+	}
+	if err := tw.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	r := &archiveReader{tar: io.NopCloser(&buf)}
+	got, err := io.ReadAll(r)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if !bytes.Equal(got, payload) {
+		t.Fatalf("payload mismatch: %q", got)
+	}
+}