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>

client/admin.html

@@ -37,6 +37,7 @@
                 <a href="#" data-panel="users" class="nav-link">Users</a>
                 <a href="#" data-panel="games" class="nav-link">Games</a>
                 <a href="#" data-panel="invites" class="nav-link">Invites</a>
+                <a href="#" data-panel="invite-requests" class="nav-link">Requests</a>
                 <a href="#" data-panel="audit" class="nav-link">Audit Log</a>
             </div>
             <div class="nav-user">
@@ -191,6 +192,35 @@
                 </table>
             </section>
 
+            <!-- Invite Requests Panel -->
+            <section id="invite-requests-panel" class="panel hidden">
+                <h2>Invite Requests</h2>
+                <div class="panel-toolbar">
+                    <div class="filter-bar">
+                        <select id="request-status-filter">
+                            <option value="pending">Pending</option>
+                            <option value="">All</option>
+                            <option value="approved">Approved</option>
+                            <option value="denied">Denied</option>
+                        </select>
+                        <button id="request-filter-btn" class="btn">Filter</button>
+                    </div>
+                </div>
+                <table id="invite-requests-table" class="data-table">
+                    <thead>
+                        <tr>
+                            <th>Name</th>
+                            <th>Email</th>
+                            <th>Message</th>
+                            <th>Submitted</th>
+                            <th>Status</th>
+                            <th>Actions</th>
+                        </tr>
+                    </thead>
+                    <tbody></tbody>
+                </table>
+            </section>
+
             <!-- Audit Log Panel -->
             <section id="audit-panel" class="panel hidden">
                 <h2>Audit Log</h2>
@@ -207,12 +237,15 @@
                             <option value="end_game">End Game</option>
                             <option value="create_invite">Create Invite</option>
                             <option value="revoke_invite">Revoke Invite</option>
+                            <option value="approve_invite_request">Approve Request</option>
+                            <option value="deny_invite_request">Deny Request</option>
                         </select>
                         <select id="audit-target-filter">
                             <option value="">All Targets</option>
                             <option value="user">Users</option>
                             <option value="game">Games</option>
                             <option value="invite_code">Invites</option>
+                            <option value="invite_request">Invite Requests</option>
                         </select>
                         <button id="audit-filter-btn" class="btn">Filter</button>
                     </div>