sync(theme-3): 同步 main 最新 API/Admin 与玩家端逻辑

从 main 同步站内信手动发送、媒体库选择、列表子路由重构等 API/Admin 改动；玩家端仅合并 ADMIN_CUSTOM 富文本、消息预览与充值截图压缩逻辑，保留 theme-3 样式。

apps/api/prisma/schema.prisma

@@ -816,22 +816,44 @@ model DepositOrderAuditLog {
 }
 
 model PlayerMessage {
-  id        BigInt    @id @default(autoincrement())
-  userId    BigInt    @map("user_id")
-  type      String    @db.VarChar(32)
-  title     String    @db.VarChar(256)
-  body      String    @db.Text
-  payload   Json?
-  readAt    DateTime? @map("read_at")
-  createdAt DateTime  @default(now()) @map("created_at")
+  id          BigInt    @id @default(autoincrement())
+  userId      BigInt    @map("user_id")
+  type        String    @db.VarChar(32)
+  title       String    @db.VarChar(256)
+  body        String    @db.Text
+  payload     Json?
+  broadcastId BigInt?   @map("broadcast_id")
+  readAt      DateTime? @map("read_at")
+  createdAt   DateTime  @default(now()) @map("created_at")
 
-  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+  user      User                     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  broadcast PlayerMessageBroadcast? @relation(fields: [broadcastId], references: [id], onDelete: Cascade)
 
   @@index([userId, createdAt(sort: Desc)])
   @@index([userId, readAt])
+  @@index([broadcastId])
   @@map("player_messages")
 }
 
+model PlayerMessageBroadcast {
+  id                BigInt   @id @default(autoincrement())
+  title             String   @db.VarChar(256)
+  body              String   @db.Text
+  translations      Json?    @map("translations")
+  targetType        String   @map("target_type") @db.VarChar(16)
+  targetUserId      BigInt?  @map("target_user_id")
+  targetUsername    String?  @map("target_username") @db.VarChar(64)
+  recipientCount    Int      @default(0) @map("recipient_count")
+  createdById       BigInt?  @map("created_by_id")
+  createdByUsername String?  @map("created_by_username") @db.VarChar(64)
+  createdAt         DateTime @default(now()) @map("created_at")
+
+  messages PlayerMessage[]
+
+  @@index([createdAt(sort: Desc)])
+  @@map("player_message_broadcasts")
+}
+
 // ============ System Config & Audit ============
 
 model SystemConfig {