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>

.env.example

@@ -75,6 +75,9 @@ SECRET_KEY=
 # Enable invite-only mode (requires invitation to register)
 INVITE_ONLY=true
 
+# Allow visitors to request an invite from the login page (only relevant when INVITE_ONLY=true)
+INVITE_REQUEST_ENABLED=false
+
 # Metered open signups (public beta)
 # 0 = disabled (invite-only enforced), -1 = unlimited, N = max open signups per day
 # When set > 0, users can register without an invite code up to the daily limit.