Admin: wall fallback ads editor (House ads pane) + /api/admin/wall-ads

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/admin.html

@@ -199,6 +199,16 @@
           <div class="card-head"><h3>House ads</h3><span class="sub" id="houseSub">running free</span></div>
           <div class="tablewrap"><table class="adm-table" id="houseTable"></table></div>
         </div>
+        <div class="card">
+          <div class="card-head"><h3>Wall fallback ads</h3><span class="sub" id="wallAdsSub">shown on member walls in positions they have not earned or filled, when no upline banner exists</span></div>
+          <p class="small muted">A member's wall has three positions. Position 1 is theirs. Positions 2 and 3 show an upline's banner until the member earns them (2 and 5 qualifying buyers); when there is no upline banner, one of these ads shows instead. They rotate in order across walls. Leave the list empty to fall back to a plain InstantAdPay card.</p>
+          <div id="wallAdsList" class="drip-steps"></div>
+          <p style="display:flex;gap:10px;flex-wrap:wrap;margin:12px 0 0">
+            <button class="btn small" id="wallAdsSave" type="button">Save wall ads</button>
+            <button class="btn small sec" id="wallAdsAdd" type="button">+ Add a wall ad</button>
+          </p>
+          <p id="wallAdsErr" class="small" style="color:var(--bad)" hidden></p>
+        </div>
       </div>
 
       <div class="pane" id="pane-campaigns" hidden>
@@ -271,6 +281,6 @@
 </div>
 
 <script src="/assets/common.js?v=20260909a"></script>
-<script src="/assets/admin.js?v=20260909c"></script>
+<script src="/assets/admin.js?v=20260909d"></script>
 </body>
 </html>