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

04 - Flows/Purchase Request Flow.md

@@ -118,7 +118,8 @@ sequenceDiagram
     B->>FE: Click "Publish"
     FE->>BE: POST /api/marketplace/purchase-requests
     BE->>DB: Duplicate check (same title+desc in 5m?)
-    BE->>BE: clean preferredSellerIds; compute isPublic
+    BE->>BE: clean preferredSellerIds
+    BE->>BE: compute isPublic
     BE->>DB: PurchaseRequest.create({status: "pending"})
     DB-->>BE: savedRequest
     BE->>N: notifyPurchaseRequestCreated(buyer, requestId)