Refuse an unversioned database that already has a files table (closes #11)

A database at user_version 0 that already has a files table was not
created by sfdupes: a foreign or partially initialized file. scan used
to run its CREATE TABLE on it and fail with a raw SQLite error, and
report and trees gave only a bare version mismatch. All three now
check for a files table at version 0 and refuse such a database with
the schema-version error telling the operator to remove the file and
rescan. A genuinely empty database is unchanged: scan creates the
schema, report and trees refuse it as before.

Model: opus-4-8 (implementation); opus-5-5 (rebase)

db.go

@@ -219,6 +219,12 @@ func openReportDatabase(ctx context.Context,
 	}
 
 	v, err := userVersion(ctx, db)
+	if err == nil && v == 0 {
+		// An empty database passes this check and fails the version
+		// check below.
+		err = checkUnversioned(ctx, db)
+	}
+
 	if err != nil {
 		_ = db.Close()
 
@@ -245,6 +251,11 @@ func initSchema(ctx context.Context, db *sql.DB) error {
 
 	switch v {
 	case 0:
+		err = checkUnversioned(ctx, db)
+		if err != nil {
+			return err
+		}
+
 		return createSchema(ctx, db)
 	case schemaVersion:
 		return nil
@@ -254,6 +265,31 @@ func initSchema(ctx context.Context, db *sql.DB) error {
 	}
 }
 
+// checkUnversioned checks a database at user_version 0 before it is
+// taken for an empty one. A database that already has a files table
+// was not created by this build: a foreign or partially initialized
+// file. Adopting it could corrupt unrelated data, so that is a
+// schema-version error telling the operator to remove the file and
+// rescan.
+func checkUnversioned(ctx context.Context, db *sql.DB) error {
+	var name string
+
+	err := db.QueryRowContext(ctx,
+		"SELECT name FROM sqlite_master "+
+			"WHERE type = 'table' AND name = 'files'").Scan(&name)
+
+	switch {
+	case err == nil:
+		return fmt.Errorf(
+			"has a files table but no schema version; "+
+				"remove the file and rescan: %w", errSchemaVersion)
+	case errors.Is(err, sql.ErrNoRows):
+		return nil
+	default:
+		return fmt.Errorf("check for files table: %w", err)
+	}
+}
+
 // createSchema applies the schema to a fresh database and stamps the
 // schema version.
 func createSchema(ctx context.Context, db *sql.DB) error {