feat: Add challenge invitations fields and fix feed hasFollowing query

- Add invitations query fields with invitee data to GetChallenge
- Add participantCount field to GetChallenge
- Add GetMyChallengeEntries query (was missing)
- Add hasFollowing to GetVideoFeed response (fixes feed ordering)
- Sync schema with backend challenge types

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/schema.gql

@@ -319,6 +319,8 @@ type Challenge {
   updatedAt: DateTime!
   ruleSet: RuleSet!
   createdBy: UserGQL!
+  invitations: [ChallengeInvitation!]!
+  participantCount: Int!
 }
 
 type RuleSet {
@@ -652,6 +654,7 @@ type ChallengeInvitation {
   createdAt: DateTime!
   challenge: Challenge!
   inviter: UserGQL!
+  invitee: UserGQL!
 }
 
 type DeployedConfigGQL {