feat: mock payment flow + flash token page (test mode)

- Mock purchase: order create → mock-pay → FlashToken issued instantly (no real billing)
- Flash page (/flash/:token): esp-web-tools integration, token state display, consume on complete
- Orders route: create/mock-pay/me/refund with full audit logging
- Flash route: GET validate, GET manifest (esp-web-tools compatible), POST consume
- MinIO file proxy (/api/files/*): browser CORS solved, firmware served through backend
- Schema: chipFamily on Project, flashOffset on ProjectFile
- ProjectNew: chipFamily selector + firmware flash offset option
- MyOrders: real order list with flash token status and buttons
- Dockerfile: prisma db push (no migration files needed for dev)
- TOSS_PAYMENT_GUIDE.md: step-by-step guide for real payment after business registration

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

platform/backend/prisma/schema.prisma

@@ -60,6 +60,7 @@ model Project {
   title           String
   description     String        @db.Text
   difficultyLevel Int           @default(3)
+  chipFamily      String        @default("ESP32-S3")
   requiredParts   Json?
   status          ProjectStatus @default(draft)
   adminNote       String?
@@ -81,6 +82,7 @@ model ProjectFile {
   fileSize     Int
   mimeType     String
   originalName String?
+  flashOffset  String   @default("0x0")
   displayOrder Int      @default(0)
   createdAt    DateTime @default(now())
 }