Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/backup/fdbbackup.go

@@ -0,0 +1,63 @@
+package backup
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+)
+
+// FDBOptions configures a FoundationDB backup via fdbbackup, FDB's own
+// backup CLI (not something Stalwart-specific) - see ARCHITECTURE.md §4.2.
+type FDBOptions struct {
+	ClusterFile string // -C; empty uses fdbbackup's own default cluster file
+	Destination string // -d, a backup URL e.g. "file:///var/backups/stalwart-fdb"
+	Tag         string // -t; defaults to "default" if empty
+}
+
+func (o FDBOptions) tag() string {
+	if o.Tag == "" {
+		return "default"
+	}
+	return o.Tag
+}
+
+// BuildFDBBackupStartArgs returns the fdbbackup argv for starting a backup,
+// without the leading "fdbbackup".
+func BuildFDBBackupStartArgs(o FDBOptions) []string {
+	args := []string{"start"}
+	if o.ClusterFile != "" {
+		args = append(args, "-C", o.ClusterFile)
+	}
+	args = append(args, "-d", o.Destination, "-t", o.tag())
+	return args
+}
+
+// StartFDBBackup kicks off an fdbbackup run. fdbbackup start returns as soon
+// as the backup job is registered, not when it finishes - callers that need
+// to know it's done should poll FDBBackupStatus and look for their own
+// definition of "complete" in its output, since this wrapper doesn't parse
+// fdbbackup's status text (that format isn't stable enough here to depend
+// on without verifying it against the fdbbackup version actually in use).
+func StartFDBBackup(ctx context.Context, o FDBOptions) error {
+	cmd := exec.CommandContext(ctx, "fdbbackup", BuildFDBBackupStartArgs(o)...)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("backup: fdbbackup start failed: %w (output: %s)", err, out)
+	}
+	return nil
+}
+
+// FDBBackupStatus returns fdbbackup's raw status output for the given tag,
+// for a human or a caller-supplied parser to interpret.
+func FDBBackupStatus(ctx context.Context, o FDBOptions) (string, error) {
+	args := []string{"status", "-t", o.tag()}
+	if o.ClusterFile != "" {
+		args = append(args, "-C", o.ClusterFile)
+	}
+	cmd := exec.CommandContext(ctx, "fdbbackup", args...)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return string(out), fmt.Errorf("backup: fdbbackup status failed: %w (output: %s)", err, out)
+	}
+	return string(out), nil
+}