feat(dashboard): add unread count for match chats

Track unread messages in match chats and display count badge:
- Schema: Add user1LastReadAt/user2LastReadAt to Match model
- Backend: Calculate unreadCount in dashboard API
- Socket: Update lastReadAt when user joins match room
- Frontend: Display red badge with unread count on match avatar

backend/prisma/migrations/20251121204228_add_match_last_read_timestamps/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable
+ALTER TABLE "matches" ADD COLUMN     "user1_last_read_at" TIMESTAMP(3),
+ADD COLUMN     "user2_last_read_at" TIMESTAMP(3),
+ALTER COLUMN "slug" DROP DEFAULT;

backend/prisma/schema.prisma

@@ -134,14 +134,16 @@ model Message {
 
 // Matches (pairs of users for collaboration)
 model Match {
-  id        Int       @id @default(autoincrement())
-  slug      String    @unique @default(cuid()) @db.VarChar(50)
-  user1Id   Int       @map("user1_id")
-  user2Id   Int       @map("user2_id")
-  eventId   Int       @map("event_id")
-  roomId    Int?      @map("room_id")
-  status    String    @default("pending") @db.VarChar(20) // 'pending', 'accepted', 'completed'
-  createdAt DateTime  @default(now()) @map("created_at")
+  id              Int       @id @default(autoincrement())
+  slug            String    @unique @default(cuid()) @db.VarChar(50)
+  user1Id         Int       @map("user1_id")
+  user2Id         Int       @map("user2_id")
+  eventId         Int       @map("event_id")
+  roomId          Int?      @map("room_id")
+  status          String    @default("pending") @db.VarChar(20) // 'pending', 'accepted', 'completed'
+  createdAt       DateTime  @default(now()) @map("created_at")
+  user1LastReadAt DateTime? @map("user1_last_read_at")
+  user2LastReadAt DateTime? @map("user2_last_read_at")
 
   // Relations
   user1     User      @relation("MatchUser1", fields: [user1Id], references: [id])