feat(db): add missing FK indexes on Listing, Payment, Subscription

- Add @@index([sellerId]) and @@index([propertyId]) to Listing model
- Add @@index([transactionId]) to Payment model
- Add @@index([planId]) to Subscription model
- Prevents full table scans on frequently-queried foreign keys

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/schema.prisma

@@ -242,6 +242,8 @@ model Listing {
   @@index([status])
   @@index([transactionType])
   @@index([priceVND])
+  @@index([sellerId])
+  @@index([propertyId])
   @@index([agentId])
   @@index([publishedAt])
 }
@@ -372,6 +374,7 @@ model Payment {
   updatedAt     DateTime        @updatedAt
 
   @@index([userId])
+  @@index([transactionId])
   @@index([status])
   @@index([providerTxId])
 }
@@ -423,6 +426,7 @@ model Subscription {
 
   usageRecords UsageRecord[]
 
+  @@index([planId])
   @@index([status])
 }