Add models caching and management functionality with corresponding API endpoints and templates

Co-authored-by: Copilot <copilot@github.com>

frontend/app/templates/generate_image.html

@@ -5,9 +5,17 @@
   <h1>Image Generation</h1>
   <form method="post" enctype="multipart/form-data">
     <label for="model">Model</label>
+    {% if models %}
+    <select id="model" name="model" required>
+      {% for m in models %}
+      <option value="{{ m.id }}" {% if request.form.get('model', '') == m.id %}selected{% endif %}>{{ m.name }}</option>
+      {% endfor %}
+    </select>
+    {% else %}
     <input id="model" name="model" type="text" required
            placeholder="e.g. openai/dall-e-3"
            value="{{ request.form.get('model', '') }}">
+    {% endif %}
 
     <label for="prompt">Prompt</label>
     <textarea id="prompt" name="prompt" rows="4" required