feat: add model availability monitoring for OpenCode fallback chain

- Add background job that checks model availability every 5 hours
- Automatically removes unavailable models from fallback chain
- Adds unavailable models section in admin UI with blur styling
- Allows admins to re-add models when they become available again
- Extends model schema with available, lastChecked, unavailableSince fields
- Adds API endpoints: GET /api/admin/models/availability, POST /api/admin/models/:id/readd

chat/public/admin.html

@@ -160,6 +160,16 @@
       <div id="opencode-models-list" class="admin-list"></div>
     </div>
 
+    <!-- Unavailable Models Section -->
+    <div class="admin-card unavailable-models-section" style="margin-top: 16px; display: none;">
+      <header>
+        <h3>Unavailable Models</h3>
+        <div class="pill" id="unavailable-models-count">0</div>
+      </header>
+      <p class="muted" style="margin-top:0;">These models were removed from the fallback chain because they are no longer available from OpenCode. You can re-add them if they become available again.</p>
+      <div id="unavailable-models-list" class="admin-list"></div>
+    </div>
+
     <!-- Public Models List with Ordering -->
     <div class="admin-card" style="margin-top: 16px;">
       <header>