feat: add community role grant and join prompt for players

- Grant Elysian Discord role to players on OAuth login (new and returning)
- Add inGuild flag to Player schema, seeded from role grant response
- Connect Discord Gateway WebSocket to keep inGuild in sync on join/leave
- Return inGuild from load endpoint; expose in game context
- Show join community modal once per session when inGuild is false

apps/api/prisma/schema.prisma

@@ -35,6 +35,7 @@ model Player {
   lifetimeAchievementsUnlocked Float @default(0)
   lastLoginDate String?
   loginStreak   Int     @default(1)
+  inGuild       Boolean @default(false)
 }
 
 model GameState {