fix(database): make cross-db migration lossless, transactional, and pre-checked

migrationModels was missing ClientGroup and ClientGlobalTraffic, so both
migration directions silently dropped client groups and global client
traffic; the model list is now extracted to allModels and a parity test
keeps the two lists from drifting again. MigrateData runs its truncate
and copy inside one transaction so a failed import rolls back instead of
leaving the destination truncated (sequences resync after commit since
setval is non-transactional). New PrepareSQLiteForMigration rejects
uploads that are not a panel database and AutoMigrates old backups so
their missing tables cannot break the row copy.

internal/database/migration_models_test.go

@@ -0,0 +1,29 @@
+package database
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestMigrationModelsMatchPanelModels(t *testing.T) {
+	names := func(models []any) map[string]bool {
+		set := make(map[string]bool, len(models))
+		for _, m := range models {
+			set[reflect.TypeOf(m).Elem().Name()] = true
+		}
+		return set
+	}
+	panel := names(allModels())
+	migration := names(migrationModels())
+
+	for name := range panel {
+		if !migration[name] {
+			t.Errorf("model %s is in allModels but missing from migrationModels: cross-db migration silently drops its rows", name)
+		}
+	}
+	for name := range migration {
+		if !panel[name] {
+			t.Errorf("model %s is in migrationModels but missing from allModels: its table never exists on a live panel", name)
+		}
+	}
+}