fix default callback base URL port mismatch

Derive CALLBACK_BASE_URL fallback from LISTEN_ADDR so default local setup resolves to localhost:8080 and stays aligned with server bind port.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/ai_client/client.go

@@ -133,7 +133,7 @@ func (d *Dispatcher) Dispatch(task *database.Task) {
 func buildCallbackURL(base string) string {
 	trimmed := strings.TrimRight(base, "/")
 	if trimmed == "" {
-		trimmed = "http://localhost"
+		trimmed = "http://localhost:8080"
 	}
 	return trimmed + "/callback"
 }