fix: implement migration tracking and make 0002 idempotent

- Add schema_migrations table to track applied migrations
- Migrate() now checks if a migration was already applied before running
- 0002 rewritten to use SAVEPOINT+ROLLBACK per column, making it safe
  to run even if some columns were partially added previously
- Each migration runs in its own transaction; INSERT into schema_migrations
  only happens if the SQL executes without error
- Bump VERSION 0.1.8 -> 0.1.9

internal/db/db.go

@@ -39,6 +39,10 @@ func (d *DB) Conn() *sql.DB {
 }
 
 func (d *DB) Migrate() error {
+	if err := d.createMigrationsTable(); err != nil {
+		return fmt.Errorf("create migrations table: %w", err)
+	}
+
 	entries, err := fs.ReadDir(migrationsFS, "migrations")
 	if err != nil {
 		return fmt.Errorf("read migrations: %w", err)
@@ -46,17 +50,75 @@ func (d *DB) Migrate() error {
 	sort.Slice(entries, func(i, j int) bool {
 		return entries[i].Name() < entries[j].Name()
 	})
+
 	for _, entry := range entries {
 		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".sql") {
 			continue
 		}
-		content, err := migrationsFS.ReadFile("migrations/" + entry.Name())
+		name := entry.Name()
+
+		applied, err := d.isMigrationApplied(name)
 		if err != nil {
-			return fmt.Errorf("read migration %s: %w", entry.Name(), err)
+			return fmt.Errorf("check migration %s: %w", name, err)
 		}
-		if _, err := d.conn.Exec(string(content)); err != nil {
-			return fmt.Errorf("apply migration %s: %w", entry.Name(), err)
+		if applied {
+			continue
+		}
+
+		content, err := migrationsFS.ReadFile("migrations/" + name)
+		if err != nil {
+			return fmt.Errorf("read migration %s: %w", name, err)
+		}
+
+		if err := d.runMigration(name, string(content)); err != nil {
+			return fmt.Errorf("apply migration %s: %w", name, err)
 		}
 	}
 	return nil
 }
+
+func (d *DB) createMigrationsTable() error {
+	_, err := d.conn.Exec(`
+		CREATE TABLE IF NOT EXISTS schema_migrations (
+			name TEXT PRIMARY KEY,
+			applied_at TEXT NOT NULL DEFAULT (datetime('now'))
+		)
+	`)
+	return err
+}
+
+func (d *DB) isMigrationApplied(name string) (bool, error) {
+	var count int
+	err := d.conn.QueryRow(
+		"SELECT COUNT(*) FROM schema_migrations WHERE name = ?",
+		name,
+	).Scan(&count)
+	if err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+func (d *DB) runMigration(name, content string) error {
+	tx, err := d.conn.Begin()
+	if err != nil {
+		return fmt.Errorf("begin transaction: %w", err)
+	}
+	defer tx.Rollback()
+
+	if _, err := tx.Exec(content); err != nil {
+		return fmt.Errorf("execute: %w", err)
+	}
+
+	if _, err := tx.Exec(
+		"INSERT INTO schema_migrations (name) VALUES (?)",
+		name,
+	); err != nil {
+		return fmt.Errorf("record migration: %w", err)
+	}
+
+	if err := tx.Commit(); err != nil {
+		return fmt.Errorf("commit: %w", err)
+	}
+	return nil
+}