fix(models): stabilize proxy endpoint refresh behavior

* fix: support large proxy model endpoint refresh

Large OpenAI-compatible proxy endpoints can expose hundreds of models and make /v1/models slow. Treating those endpoints like local model servers caused model picker opens and background probes to repeatedly hit /models, producing timeouts and making otherwise usable endpoints appear offline.

Make model endpoint discovery cached-first for normal UI usage, add explicit proxy/API classification and refresh policy fields, exclude proxy/API endpoints from aggressive local probing, and preserve cached models when refresh fails.

Manual Test/Add/Refresh actions still fetch the full model list with longer timeouts so users can intentionally import large proxy model lists without blocking normal model picker usage.

* fix: preserve endpoint ping status semantics

static/index.html

@@ -2079,6 +2079,10 @@
                 </select>
                 <div class="admin-model-form-row">
                   <input id="adm-epApiKey" type="password" placeholder="API key">
+                  <select id="adm-epKind" style="padding:5px;width:82px;">
+                    <option value="proxy">Proxy</option>
+                    <option value="api">API</option>
+                  </select>
                   <select id="adm-epType" style="padding:5px;width:80px;">
                     <option value="llm">LLM</option>
                     <option value="image">Image</option>