fix(upload): reject exact duplicates at upload time and prevent duplicate mobile uploads

- Move duplicate check before task enqueue in ui_upload endpoint
- Clean up temp file and return status "duplicate" for exact duplicates
- Add URI-level dedup guard in mobile UploadScreen to prevent repeated uploads
- Improve ShareContext URI normalization (collapse slashes, decode percent-encoding)
- Guard +not-found.tsx effect against re-firing for the same pathname
- Update mobile UploadResponse type and handlers for duplicate status
- Update web frontend upload.js to show duplicate status
- Update API and Configuration docs

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/static/js/upload.js

@@ -433,9 +433,17 @@ function _uploadSingleFile(file, progressBar, statusEl, onTerminal) {
       if (xhr.status === 200) {
         const result = JSON.parse(xhr.responseText);
         progressBar.style.width = '100%';
-        progressBar.className = 'file-progress-bar bg-green-500 h-2 rounded-full';
-        statusEl.textContent = `Success: Task ID: ${result.task_id}`;
-        statusEl.className = 'text-xs text-green-600 mt-1';
+
+        if (result.status === 'duplicate' && result.duplicate_of) {
+          // Exact duplicate – no processing task was created
+          progressBar.className = 'file-progress-bar bg-yellow-400 h-2 rounded-full';
+          statusEl.textContent = `Duplicate – already processed (file #${result.duplicate_of.original_file_id})`;
+          statusEl.className = 'text-xs text-yellow-600 mt-1';
+        } else {
+          progressBar.className = 'file-progress-bar bg-green-500 h-2 rounded-full';
+          statusEl.textContent = `Success: Task ID: ${result.task_id}`;
+          statusEl.className = 'text-xs text-green-600 mt-1';
+        }
         _onUploadSuccess();
         onTerminal();
         resolve({ rateLimited: false, retryAfterSeconds: 0 });