fix(models): make pinned models visible in chat UI (#2481)

Two bugs prevented pinned models from appearing in the chat model picker:

1. _fetch_models() only used _cached_model_ids(), ignoring pinned_models.
   Since Fireworks AI doesn't list kimi-k2p6-turbo in /v1/models, the
   cached list was empty, so the endpoint showed as offline with no models.

2. _curate_models() filtered unknown pinned IDs into models_extra, but the
   chat UI only reads models (primary list). Pinned models stayed invisible.

Fix: use _visible_models() to merge cached + pinned, then promote pinned
IDs from models_extra to models so they appear in the dropdown.

Closes #1521 follow-up

routes/model_routes.py

@@ -1029,12 +1029,13 @@ def setup_model_routes(model_discovery):
         for ep in endpoints:
             base = _normalize_base(ep.base_url)
             provider = _detect_provider(base)
-            # Use cached models — background refresh keeps them updated
-            model_ids = _cached_model_ids(ep)
+            # Merge cached + pinned models, then filter out hidden ones
             ep_model_type = getattr(ep, "model_type", None) or "llm"
-            # Filter out hidden (probe-failed) models
-            hidden = _hidden_model_ids(ep)
-            model_ids = [m for m in model_ids if m not in hidden]
+            model_ids = _visible_models(
+                _cached_model_ids(ep),
+                ep.hidden_models,
+                getattr(ep, "pinned_models", None),
+            )
             # Build correct URL based on provider
             chat_url = build_chat_url(base)
             kind = _effective_endpoint_kind(ep, base)
@@ -1043,6 +1044,13 @@ def setup_model_routes(model_discovery):
             if model_ids:
                 curated_key = _match_provider_curated(base, None)
                 curated, extra = _curate_models(model_ids, curated_key)
+                # Pinned models are admin-selected — they always belong in the
+                # primary curated list, not buried in extras.
+                pinned = _normalize_model_ids(getattr(ep, "pinned_models", None))
+                for m in pinned:
+                    if m not in curated:
+                        curated.append(m)
+                extra = [m for m in extra if m not in pinned]
                 items.append({
                     "host": "custom",
                     "port": 0,