feat: ko-fi integration for credit purchases

backend/prisma/schema.prisma

@@ -23,6 +23,7 @@ model User {
   watchEvents      WatchEvent[]
   contentRequests  ContentRequest[]
   sessions         Session[]
+  kofiPayments     KofiPayment[]
 
   createdAt DateTime @default(now()) @map("created_at")
   updatedAt DateTime @updatedAt @map("updated_at")
@@ -118,11 +119,33 @@ model SystemSettings {
   @@map("system_settings")
 }
 
+model KofiPayment {
+  id                String   @id @default(uuid())
+  kofiTransactionId String   @unique @map("kofi_transaction_id")
+  email             String
+  fromName          String   @map("from_name")
+  amount            Float
+  creditsGranted    Int      @map("credits_granted")
+  message           String?
+  isPublic          Boolean  @default(true) @map("is_public")
+  currency          String   @default("USD")
+  userId            String?  @map("user_id")
+  user              User?    @relation(fields: [userId], references: [id])
+  status            String   @default("PENDING")
+  createdAt         DateTime @default(now()) @map("created_at")
+
+  @@index([userId])
+  @@index([email])
+  @@index([status])
+  @@map("kofi_payments")
+}
+
 enum TransactionType {
   EARN
   SPEND
   BONUS
   ADJUSTMENT
+  PURCHASE
 }
 
 enum RequestStatus {