feat: add Prisma schema for pairing history

Add MongoDB schema with Prisma 6.8.2 for tracking:
- ServerEvent: events per server (coffee chats, study groups, etc.)
- UserPairingHistory: user pairing records per server
- PairingRecord: individual pairing entries with event context

This enables smart grouping that avoids recent pairings per-event.

prisma/schema.prisma

@@ -0,0 +1,35 @@
+generator client {
+    provider = "prisma-client-js"
+}
+
+datasource db {
+    provider = "mongodb"
+    url      = env("MONGO_URI")
+}
+
+model ServerEvent {
+    id        String   @id @default(auto()) @map("_id") @db.ObjectId
+    serverId  String
+    eventId   String
+    eventName String
+    createdAt DateTime @default(now())
+    createdBy String
+
+    @@unique([serverId, eventId])
+    @@index([serverId])
+}
+
+model UserPairingHistory {
+    id       String          @id @default(auto()) @map("_id") @db.ObjectId
+    userId   String
+    serverId String
+    pairings PairingRecord[]
+
+    @@unique([userId, serverId])
+}
+
+type PairingRecord {
+    recipientId String
+    eventId     String
+    sessionDate DateTime
+}