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/styles.css

@@ -1161,4 +1161,29 @@ textarea:focus {
   font-size: 14px;
   font-weight: 600;
   letter-spacing: 0.02em;
+}
+
+/* Unavailable models styling */
+.unavailable-models-section {
+  background: rgba(176, 0, 32, 0.02);
+  border-color: rgba(176, 0, 32, 0.15);
+}
+
+.unavailable-model {
+  opacity: 0.7;
+  filter: blur(0.5px);
+  background: #fafafa;
+}
+
+.unavailable-model:hover {
+  opacity: 1;
+  filter: none;
+}
+
+.unavailable-model .model-chip {
+  filter: grayscale(30%);
+}
+
+.unavailable-model img {
+  filter: grayscale(100%);
 }