feat: track event participation and show joined events first

Backend:
- Add EventParticipant model to track user-event participation
- Create database migration for event_participants table
- Record participation when user joins event chat via Socket.IO
- Update GET /api/events to include isJoined flag for current user
- Sort events: joined events first, then by start date
- Add authenticate middleware to GET /api/events

Frontend:
- Replace mock events with real API data from backend
- Add loading and error states to EventsPage
- Display "Joined" badge on events user has joined
- Highlight joined events with colored border
- Show "Open chat" vs "Join chat" button text
- Auto-refresh events list when navigating back

When users join an event chat, this is now recorded in the database.
Joined events appear at the top of the list with visual indicators.

backend/prisma/schema.prisma

@@ -53,6 +53,7 @@ model User {
   matchesAsUser2          Match[]  @relation("MatchUser2")
   ratingsGiven            Rating[]  @relation("RaterRatings")
   ratingsReceived         Rating[] @relation("RatedRatings")
+  eventParticipants       EventParticipant[]
 
   @@map("users")
 }
@@ -72,6 +73,7 @@ model Event {
   // Relations
   chatRooms         ChatRoom[]
   matches           Match[]
+  participants      EventParticipant[]
 
   @@map("events")
 }
@@ -153,3 +155,20 @@ model Rating {
   @@index([ratedId])
   @@map("ratings")
 }
+
+// Event participants (tracks which users joined which events)
+model EventParticipant {
+  id        Int      @id @default(autoincrement())
+  userId    Int      @map("user_id")
+  eventId   Int      @map("event_id")
+  joinedAt  DateTime @default(now()) @map("joined_at")
+
+  // Relations
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  event     Event    @relation(fields: [eventId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, eventId])
+  @@index([userId])
+  @@index([eventId])
+  @@map("event_participants")
+}