fix: restrict CORS to configured origins (closes #40)

- Add CORSOrigins config field (UPAAS_CORS_ORIGINS env var)
- Default to same-origin only (no CORS headers when unconfigured)
- When configured, allow specified origins with AllowCredentials: true
- Add tests for CORS middleware behavior

internal/service/deploy/deploy.go

@@ -82,7 +82,7 @@ type deploymentLogWriter struct {
 	lineBuffer bytes.Buffer // buffer for incomplete lines
 	mu         sync.Mutex
 	done       chan struct{}
-	flushed    sync.WaitGroup // waits for flush goroutine to finish
+	flushed    sync.WaitGroup  // waits for flush goroutine to finish
 	flushCtx   context.Context //nolint:containedctx // needed for async flush goroutine
 }