admin: submissions page, download endpoint, uploads search by app_id

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

internal/admin/web/cluster.html

@@ -117,10 +117,11 @@
   <header>
     <div style='display:flex;align-items:center;gap:1rem;'>
       <h1>spark-mcp-go Admin</h1>
-      <nav>
-        <a href='/admin' class='active'>Clusters</a>
-        <a href='/admin/uploads'>Uploads</a>
-      </nav>
+     <nav>
+       <a href='/admin' class='active'>Clusters</a>
+       <a href='/admin/uploads'>Uploads</a>
+        <a href='/admin/submissions'>Submissions</a>
+     </nav>
     </div>
     <div class='auth'>
       <button id='set-token' class='secondary'>Set Token</button>