feat: host_type field for OpenRouter / OpenAI-compatible API support

Adds host_type ("openwebui" | "openai") to the host schema so Cortex can
talk to both Open WebUI/Ollama and OpenRouter/standard-OpenAI endpoints.

Path differences per type:
  openwebui (default): /api/chat/completions, /api/models
  openai:              /chat/completions,     /models

model_registry.py:
  - host_type added to host schema (default "openwebui", backward compat)
  - save_host() accepts host_type parameter
  - _resolve_model() passes host_type through with the merged host fields

llm_client._local():
  - Reads host_type from resolved model_cfg
  - Selects correct chat completions path accordingly

routers/local_llm.py:
  - save_host route accepts host_type form field
  - fetch-models uses /models for openai type, /api/models for openwebui
  - Existing host rows show type selector pre-filled from stored value

local_llm.html:
  - "Add host" form includes type selector

To use OpenRouter:
  - Add host: URL = https://openrouter.ai/api/v1, Type = OpenAI-compatible
  - API key from openrouter.ai (store in .env or model_registry.json only)
  - Fetch models or add manually (e.g. anthropic/claude-sonnet-4-5-20251022)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/local_llm.html

@@ -243,11 +243,20 @@
                        autocomplete="off" spellcheck="false" data-form-type="other">
               </div>
             </div>
-            <div class="field">
-              <label for="new-host-key">API Key</label>
-              <input type="password" id="new-host-key" name="api_key"
-                     placeholder="sk-… (leave blank if not required)"
-                     autocomplete="new-password" data-1p-ignore data-lpignore="true" data-form-type="other">
+            <div class="field-row">
+              <div class="field">
+                <label for="new-host-key">API Key</label>
+                <input type="password" id="new-host-key" name="api_key"
+                       placeholder="sk-… (leave blank if not required)"
+                       autocomplete="new-password" data-1p-ignore data-lpignore="true" data-form-type="other">
+              </div>
+              <div class="field" style="flex:0 0 auto">
+                <label for="new-host-type">Type</label>
+                <select id="new-host-type" name="host_type">
+                  <option value="openwebui">Open WebUI / Ollama</option>
+                  <option value="openai">OpenAI-compatible (OpenRouter, etc.)</option>
+                </select>
+              </div>
             </div>
             <div class="btn-row">
               <button type="submit" class="btn btn-primary btn-sm">Add Host</button>