feat: add match slugs for security and fix message history loading

Security improvements:
- Add random CUID slugs to Match model to prevent ID enumeration attacks
- Update all match URLs from /matches/:id to /matches/:slug
- Keep numeric IDs for internal Socket.IO operations only

Backend changes:
- Add slug field to matches table with unique index
- Update all match endpoints to use slug-based lookups (GET, PUT, DELETE)
- Add GET /api/matches/:slug/messages endpoint to fetch message history
- Include matchSlug in all Socket.IO notifications

Frontend changes:
- Update all match routes to use slug parameter
- Update MatchesPage to use slug for accept/reject/navigate operations
- Update MatchChatPage to fetch match data by slug and load message history
- Update RatePartnerPage to use slug parameter
- Add matchesAPI.getMatchMessages() function

Bug fixes:
- Fix MatchChatPage not loading message history from database on mount
- Messages now persist and display correctly when users reconnect

backend/prisma/schema.prisma

@@ -131,6 +131,7 @@ 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")