Add admin LLM model setting; rename Pi to LLM in the UI; drop table Review column

- New server/llm.js consolidates ModelRuntime + model selection: admin
  setting (app_settings.llm_model) > LLM_MODEL/PREFILL_MODEL env > first
  available; used by both prefill and roast evaluation
- GET/PUT /api/admin/llm lists configured models and stores the choice
  (validated against the list; empty = auto; audited); admin page gains
  an LLM section with a model picker
- All user-facing 'Pi agent' wording is now 'LLM'; no_model error message
  no longer references the pi CLI
- /roasts table: Review column removed (review lives in the detail view)

Co-Authored-By: Claude Fable 5 <[email protected]>

public/roasts.html

@@ -82,7 +82,7 @@
               <div class="brand-text">
                 <h1>Roast history</h1>
                 <p class="brand-sub">
-                  Finished roasts uploaded from Artisan, reviewed by the Pi agent
+                  Finished roasts uploaded from Artisan, with LLM reviews
                 </p>
               </div>
             </div>
@@ -114,7 +114,6 @@
                       <th>Drop</th>
                       <th>DTR</th>
                       <th>Loss</th>
-                      <th>Review</th>
                     </tr>
                   </thead>
                   <tbody id="roasts-body"></tbody>