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;