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/preflight/storebackend_test.go

@@ -0,0 +1,86 @@
+package preflight
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDetectStoreBackendsTOML(t *testing.T) {
+	toml := `
+[server]
+hostname = "mail.example.com"
+
+[store."rocksdb"]
+type = "rocksdb"
+path = "/var/lib/stalwart/data"
+
+[store."blob"]
+type = "s3"
+bucket = "stalwart-blobs"
+`
+	path := filepath.Join(t.TempDir(), "config.toml")
+	if err := os.WriteFile(path, []byte(toml), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	matches, err := DetectStoreBackends(path)
+	if err != nil {
+		t.Fatalf("DetectStoreBackends: %v", err)
+	}
+	if len(matches) != 2 {
+		t.Fatalf("got %d matches, want 2: %+v", len(matches), matches)
+	}
+	got := map[string]string{}
+	for _, m := range matches {
+		got[m.Path] = m.Backend
+	}
+	if got[`store."rocksdb"`] != "rocksdb" {
+		t.Errorf(`store."rocksdb" backend = %q, want rocksdb`, got[`store."rocksdb"`])
+	}
+	if got[`store."blob"`] != "s3" {
+		t.Errorf(`store."blob" backend = %q, want s3`, got[`store."blob"`])
+	}
+}
+
+func TestDetectStoreBackendsJSON(t *testing.T) {
+	jsonCfg := `{
+		"store": {
+			"data": {"type": "postgresql", "host": "db.internal"},
+			"blob": {"type": "s3", "bucket": "stalwart-blobs"}
+		}
+	}`
+	path := filepath.Join(t.TempDir(), "config.json")
+	if err := os.WriteFile(path, []byte(jsonCfg), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	matches, err := DetectStoreBackends(path)
+	if err != nil {
+		t.Fatalf("DetectStoreBackends: %v", err)
+	}
+	if len(matches) != 2 {
+		t.Fatalf("got %d matches, want 2: %+v", len(matches), matches)
+	}
+	backends := map[string]bool{}
+	for _, m := range matches {
+		backends[m.Backend] = true
+	}
+	if !backends["postgresql"] || !backends["s3"] {
+		t.Errorf("matches = %+v, want postgresql and s3", matches)
+	}
+}
+
+func TestDetectStoreBackendsNoMatch(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.toml")
+	if err := os.WriteFile(path, []byte("[server]\nhostname = \"mail.example.com\"\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	matches, err := DetectStoreBackends(path)
+	if err != nil {
+		t.Fatalf("DetectStoreBackends: %v", err)
+	}
+	if len(matches) != 0 {
+		t.Errorf("got %d matches, want 0: %+v", len(matches), matches)
+	}
+}