feat: setup the attachment to the passenger api

apps/edr-passenger-api/prisma/schema.prisma

@@ -916,10 +916,37 @@ model SupportMessage {
   id             String              @id @default(uuid())
   conversationId String
   sender         SupportSender
-  text           String
-  attachments    Json?
+  /// NULL for an attachment-only message — absence of text is representable
+  /// rather than smuggled through "". The DTO maps NULL -> "".
+  text           String?
   createdAt      DateTime            @default(now())
   conversation   SupportConversation @relation(fields: [conversationId], references: [id])
+  attachments    SupportAttachment[]
+  /// Backs keyset pagination of a thread (newest-first over (createdAt, id)).
+  /// Without it, paging a long thread degrades to a scan per page.
+  @@index([conversationId, createdAt])
+  @@schema("passenger")
+}
+
+/// A file posted on a support message.
+///
+/// The freight side stores the equivalent in its polymorphic `freight.files`
+/// table; this app has no such table (and no TypeORM), so chat attachments get a
+/// purpose-built model rather than a shared one. Bytes live in MinIO — `url` is
+/// the unsigned object path, signed on read for preview.
+model SupportAttachment {
+  id        String         @id @default(uuid())
+  messageId String
+  name      String
+  mimeType  String
+  /// Bytes.
+  size      Int
+  /// Unsigned MinIO object URL. Not directly fetchable by a browser — the API
+  /// mints a short-lived signed URL per response.
+  url       String
+  createdAt DateTime       @default(now())
+  message   SupportMessage @relation(fields: [messageId], references: [id], onDelete: Cascade)
+  @@index([messageId])
   @@schema("passenger")
 }