feat(sync): plumb context_text through SaveProgress and sync queue

- SaveProgressRequest: add ContextText field
- SaveProgress: carry existing ContextText from DB, overwrite when provided
- ProgressUpdate: add ContextText field for checkpoint sync
- SyncQueueProcessor: serialize/deserialize context_text in sync data
- buildProgressSnapshot: include context_text in snapshot data

internal/sync/queue.go

@@ -48,6 +48,7 @@ type ProgressUpdate struct {
 	UserID      pgtype.UUID
 	Percentage  float64
 	Epubcfi     *string
+	ContextText *string
 	Chapter     *int
 	Character   *int64
 	Page        *int
@@ -122,6 +123,9 @@ func (p *SyncQueueProcessor) enqueueProgressUpdate(ctx context.Context, update *
 	if update.Epubcfi != nil {
 		syncData["epubcfi"] = *update.Epubcfi
 	}
+	if update.ContextText != nil {
+		syncData["context_text"] = *update.ContextText
+	}
 	if update.Chapter != nil {
 		syncData["chapter"] = *update.Chapter
 	}
@@ -331,6 +335,9 @@ func (p *SyncQueueProcessor) syncProgress(ctx context.Context, userID pgtype.UUI
 		if v, ok := syncData["epubcfi"].(string); ok {
 			req.Epubcfi = &v
 		}
+		if v, ok := syncData["context_text"].(string); ok {
+			req.ContextText = &v
+		}
 		if v, ok := syncData["chapter"].(float64); ok {
 			ch := int(v)
 			req.Chapter = &ch