fix: sqlite concurrency defaults

internal/db/sqlite.go

@@ -10,9 +10,14 @@ import (
 
 // Open connects to a sqlite3 database with foreign keys enforced
 func Open(dbFile string) (*sql.DB, error) {
-	db, err := sql.Open("sqlite3", fmt.Sprintf("file:%s?_foreign_keys=on", dbFile))
+	// busy_timeout avoids immediate SQLITE_BUSY errors under concurrent access.
+	// foreign_keys ensures FK constraints are enforced for this connection.
+	db, err := sql.Open("sqlite3", fmt.Sprintf("file:%s?_foreign_keys=on&_busy_timeout=5000", dbFile))
 	if err != nil {
 		return nil, fmt.Errorf("failed to open db: %w", err)
 	}
+	// WAL improves concurrency between readers and writers.
+	_, _ = db.Exec("PRAGMA journal_mode=WAL;")
+	_, _ = db.Exec("PRAGMA busy_timeout=5000;")
 	return db, nil
 }