Add invite request system and Gitea Actions CI/CD pipeline

Invite request feature:
- Public form to request an invite when INVITE_REQUEST_ENABLED=true
- Stores requests in new invite_requests DB table
- Emails admins on new request, emails requester on approve/deny
- Admin panel tab to review, approve, and deny requests
- Approval auto-creates invite code and sends signup link

CI/CD pipeline:
- Build & push Docker image to Gitea registry on release
- Auto-deploy to staging with health check
- Manual workflow_dispatch for production deploys

Also includes client layout/sizing improvements for card grid
and opponent spacing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.staging.yml

@@ -29,6 +29,7 @@ services:
       - BASE_URL=${BASE_URL:-https://staging.golfcards.club}
       - RATE_LIMIT_ENABLED=false
       - INVITE_ONLY=true
+      - INVITE_REQUEST_ENABLED=false
       - DAILY_OPEN_SIGNUPS=${DAILY_OPEN_SIGNUPS:-0}
       - DAILY_SIGNUPS_PER_IP=${DAILY_SIGNUPS_PER_IP:-3}
       - BOOTSTRAP_ADMIN_USERNAME=${BOOTSTRAP_ADMIN_USERNAME:-}