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/admin.html

@@ -128,6 +128,27 @@
             </div>
           </section>
 
+          <section class="panel-card" id="llm-card">
+            <div class="panel-head"><h2>LLM</h2></div>
+            <div class="panel-body">
+              <p class="muted" style="margin-top:0">
+                Model used for URL prefill and finished-roast reviews.
+              </p>
+              <div style="display:flex;gap:8px;align-items:center;flex-wrap:wrap">
+                <select
+                  class="text-input"
+                  id="llm-model-select"
+                  style="max-width:360px"
+                  aria-label="LLM model"
+                ></select>
+                <button class="ghost-btn small" type="button" id="llm-model-save">
+                  Save
+                </button>
+              </div>
+              <p class="field-note" id="llm-model-note"></p>
+            </div>
+          </section>
+
           <section class="panel-card" id="users">
             <div class="panel-head">
               <h2>Users</h2>