Fix Cantaloupe Sync: use sys.executable + add Excel file upload button

- Changed subprocess from 'python' to 'sys.executable' (fixes FileNotFoundError
  in systemd — venv python not in system PATH)
- Added '📤 Upload Excel' button to admin frontend sync page
- uploadSyncFile() sends file to the same /api/admin/cantaloupe/sync endpoint
  (which already supported file upload — frontend just didn't expose it)
- Users can now use 'Sync Now' for auto-download or 'Upload Excel' for manual

cantaloupe_sync.py

@@ -419,9 +419,10 @@ async def run_sync(
         # Try subprocess call to cantaloupe export
         try:
             import subprocess
+            import sys
             cantaloupe_dir = os.path.expanduser("~/projects/cantaloupe-downloader")
             result = subprocess.run(
-                ["python", "-m", "cantaloupe", "export", "--scheduled"],
+                [sys.executable, "-m", "cantaloupe", "export", "--scheduled"],
                 capture_output=True, text=True, timeout=120,
                 cwd=cantaloupe_dir,
             )

static/index.html

@@ -864,6 +864,8 @@
       <span>Cantaloupe Sync</span>
       <div class="pt-actions">
         <button class="btn btn-sm btn-primary" onclick="runSync()" id="csSyncBtn">🔄 Sync Now</button>
+        <input type="file" id="csFileInput" accept=".xlsx,.xls" style="display:none" onchange="uploadSyncFile(event)">
+        <button class="btn btn-sm btn-outline" onclick="document.getElementById('csFileInput').click()">📤 Upload Excel</button>
         <button class="btn btn-sm btn-outline" onclick="loadCantaloupeSync()">↻ Refresh</button>
       </div>
     </div>
@@ -2320,6 +2322,28 @@ async function runSync() {
   }
 }
 
+// ── Upload Excel File ──
+async function uploadSyncFile(event) {
+  const file = event.target.files?.[0];
+  if (!file) return;
+  const fd = new FormData();
+  fd.append('file', file);
+  const btn = document.getElementById('csSyncBtn');
+  btn.disabled = true;
+  btn.innerHTML = '<div class="spinner"></div> Uploading...';
+  try {
+    const result = await api('/api/admin/cantaloupe/sync', { method: 'POST', body: fd });
+    showToast('Upload processed — batch #' + result.id);
+    await loadCantaloupeSync();
+  } catch (e) {
+    showToast('Upload failed: ' + e.message, true);
+  } finally {
+    btn.disabled = false;
+    btn.innerHTML = '🔄 Sync Now';
+    event.target.value = ''; // reset so same file can be re-picked
+  }
+}
+
 // ── Approve / Reject ──
 async function approveBatch(batchId) {
   const confirmed = await showModal('Approve Batch',