fix: address code review findings

internal/database/database.go

@@ -15,7 +15,11 @@ type DB struct {
 
 // New opens a SQLite database at dbPath and runs schema migrations.
 func New(dbPath string) (*DB, error) {
-	conn, err := sql.Open("sqlite3", dbPath)
+	// The _foreign_keys=on DSN parameter enables foreign key enforcement on
+	// EVERY connection in the pool. A one-off "PRAGMA foreign_keys=ON" executed
+	// on the pooled *sql.DB only applies to the first connection and is lost on
+	// connections opened later by the pool, silently disabling the safety net.
+	conn, err := sql.Open("sqlite3", dbPath+"?_foreign_keys=on")
 	if err != nil {
 		return nil, fmt.Errorf("open database: %w", err)
 	}
@@ -26,12 +30,6 @@ func New(dbPath string) (*DB, error) {
 		return nil, fmt.Errorf("set WAL mode: %w", err)
 	}
 
-	// Enable foreign key enforcement.
-	if _, err := conn.Exec("PRAGMA foreign_keys=ON"); err != nil {
-		conn.Close()
-		return nil, fmt.Errorf("enable foreign keys: %w", err)
-	}
-
 	// Set busy timeout to handle concurrent write contention.
 	if _, err := conn.Exec("PRAGMA busy_timeout=5000"); err != nil {
 		conn.Close()