add structured logging and admin diagnostics to serve

- bincio.serve logger wired into uvicorn output: rebuild steps, upload
  errors, strava-zip progress all now appear in the server log
- _trigger_rebuild: capture stdout/stderr, log errors instead of silently
  discarding; exceptions logged with traceback instead of swallowed
- upload handler: log per-file errors with traceback; include error detail
  in the SSE event sent back to the browser
- strava-zip handler: log imported/error counts on completion
- GET /api/admin/users/{handle}/diag: snapshot of a user's data dir
  (file counts, sizes, index activity counts, pending uploads)
- POST /api/admin/users/{handle}/rebuild-sync: blocking rebuild that
  returns full stdout/stderr — for debugging without SSH log access
- Admin page: Diag button per user opens a modal showing the diag JSON

site/src/pages/admin/index.astro

@@ -31,6 +31,15 @@ import Base from '../../layouts/Base.astro';
       </table>
     </div>
 
+    <!-- Diag modal -->
+    <dialog id="diag-dialog" class="rounded-xl bg-zinc-900 border border-zinc-700 p-6 text-white max-w-2xl w-full backdrop:bg-black/60">
+      <div class="flex items-center justify-between mb-4">
+        <h3 class="font-semibold text-sm">Data directory snapshot — <span id="diag-handle" class="text-zinc-400 font-mono"></span></h3>
+        <button id="diag-close" class="text-zinc-500 hover:text-zinc-200 text-xs px-2 py-1 rounded bg-zinc-800">Close</button>
+      </div>
+      <pre id="diag-output" class="text-xs font-mono bg-zinc-950 rounded p-4 overflow-auto max-h-96 text-green-300 whitespace-pre-wrap"></pre>
+    </dialog>
+
     <!-- Confirmation dialog -->
     <dialog id="confirm-dialog" class="rounded-xl bg-zinc-900 border border-zinc-700 p-6 text-white max-w-sm w-full backdrop:bg-black/60">
       <p class="text-sm text-zinc-300 mb-1">Reset all data for <strong id="confirm-handle" class="text-white"></strong>?</p>
@@ -44,10 +53,15 @@ import Base from '../../layouts/Base.astro';
 </Base>
 
 <script>
-  const overviewEl = document.getElementById('disk-overview')!;
-  const tbodyEl    = document.getElementById('user-list')!;
-  const dialog     = document.getElementById('confirm-dialog') as HTMLDialogElement;
-  const confirmH   = document.getElementById('confirm-handle')!;
+  const overviewEl  = document.getElementById('disk-overview')!;
+  const tbodyEl     = document.getElementById('user-list')!;
+  const dialog      = document.getElementById('confirm-dialog') as HTMLDialogElement;
+  const confirmH    = document.getElementById('confirm-handle')!;
+  const diagDialog  = document.getElementById('diag-dialog') as HTMLDialogElement;
+  const diagHandle  = document.getElementById('diag-handle')!;
+  const diagOutput  = document.getElementById('diag-output')!;
+  document.getElementById('diag-close')!.addEventListener('click', () => diagDialog.close());
+  diagDialog.addEventListener('click', e => { if (e.target === diagDialog) diagDialog.close(); });
   const confirmOk     = document.getElementById('confirm-ok')!;
   const confirmCancel = document.getElementById('confirm-cancel')!;
 
@@ -117,6 +131,11 @@ import Base from '../../layouts/Base.astro';
           <td class="px-4 py-3 text-right text-zinc-400 tabular-nums">${u.images_mb > 0 ? fmt(u.images_mb) : '—'}</td>
           <td class="px-4 py-3 text-right">
             <div class="flex gap-2 justify-end">
+              <button
+                class="diag-btn text-xs px-3 py-1.5 rounded-lg bg-zinc-800 hover:bg-zinc-700 text-zinc-400 hover:text-zinc-200 transition-colors"
+                data-handle="${u.handle}"
+                title="Show diagnostic snapshot of this user's data directory"
+              >Diag</button>
               <button
                 class="rebuild-btn text-xs px-3 py-1.5 rounded-lg bg-zinc-800 hover:bg-zinc-700 text-zinc-400 hover:text-zinc-200 transition-colors"
                 data-handle="${u.handle}"
@@ -138,6 +157,22 @@ import Base from '../../layouts/Base.astro';
       `;
     }).join('');
 
+    tbodyEl.querySelectorAll<HTMLButtonElement>('.diag-btn').forEach(btn => {
+      btn.addEventListener('click', async () => {
+        const h = btn.dataset.handle!;
+        diagHandle.textContent = h;
+        diagOutput.textContent = 'Loading…';
+        diagDialog.showModal();
+        try {
+          const r = await fetch(`/api/admin/users/${h}/diag`, { credentials: 'include' });
+          const d = await r.json();
+          diagOutput.textContent = JSON.stringify(d, null, 2);
+        } catch (err) {
+          diagOutput.textContent = String(err);
+        }
+      });
+    });
+
     tbodyEl.querySelectorAll<HTMLButtonElement>('.rebuild-btn').forEach(btn => {
       btn.addEventListener('click', async () => {
         const h = btn.dataset.handle!;