fix(platform-links): prevent duplicate displayOrder via unique constraint and retry

Concurrent createPlatformLink calls both read the same max(displayOrder)
and insert the same value, corrupting link ordering for the user.

- Add @@unique([userId, displayOrder]) to PlatformLink schema with migration
- Wrap createPlatformLink in a retry loop (max 5 attempts) that re-reads
  max and retries on P2002 unique constraint violations
- Reorder uses two-phase transaction (temp offset then final values) to
  avoid constraint conflicts when adjacent positions swap
- Add platform-link-ordering.test.ts covering concurrency, retry, two-phase
  reorder, ordering integrity, and regression scenarios

Closes #485

Author: Ridanshi

apps/backend/prisma/migrations/20260612000000_platform_link_unique_display_order/migration.sql

@@ -0,0 +1,2 @@
+-- Add unique constraint on (user_id, display_order) to prevent duplicate display orders per user
+CREATE UNIQUE INDEX "platform_links_user_id_display_order_key" ON "platform_links"("user_id", "display_order");

apps/backend/prisma/schema.prisma

@@ -92,6 +92,7 @@ model PlatformLink {
   user      User       @relation(fields: [userId], references: [id], onDelete: Cascade)
   cardLinks CardLink[]
 
+  @@unique([userId, displayOrder])
   @@map("platform_links")
 }