fix(page): serialize local image uploads to respect Notion rate limits

prepareLocalImageUploads fanned out up to four uploads in parallel,
but each upload fires three sequential official-API calls (create,
send, then poll), so even two in-flight uploads easily exceed
Notion's documented ~3 req/sec average limit. Without a shared rate
limiter or 429/Retry-After handling, multi-image sync hits HTTP 429
nondeterministically. Drop the concurrency limit to 1 so uploads
proceed serially and stay under the documented budget; full
rate-limit/backoff infrastructure can land separately.

Author: 0xble

cmd/page_local_images.go

@@ -15,8 +15,13 @@ import (
 	"github.com/lox/notion-cli/internal/output"
 )
 
-// localImageUploadConcurrency caps concurrent local image uploads.
-const localImageUploadConcurrency = 4
+// localImageUploadConcurrency caps concurrent local image uploads. Each
+// upload fires at least three sequential Notion API calls (create file
+// upload, send, then poll for upload status), so even serial work lives
+// right at Notion's documented ~3 req/sec average limit. Fanning out
+// in parallel without a shared rate limiter and 429/Retry-After backoff
+// reliably trips rate limits on pages with several images.
+const localImageUploadConcurrency = 1
 
 type uploadedLocalImage struct {
 	Alt          string