fix: repair Mermaid diagram syntax errors and add PRD task plan

04 - Flows/Referral Flow.md

@@ -93,7 +93,8 @@ sequenceDiagram
 
     Note over BE,DB: Later, when N completes a purchase
     BE->>BE: PointsService.addPoints(R, +X, 'referral', {referredUserId:N})
-    BE->>DB: user.points += X; PointTransaction.create
+    BE->>DB: add X points to user balance
+    BE->>DB: create PointTransaction record
     BE->>BE: updateUserLevel → maybe 'level-up'
     BE->>IO: emit user-{R} 'level-up'
 ```