feat(ui): show relative timestamps on Dashboard and Downloads tables

Converts the Source Health last-check column and the Downloads created-at
column from locale date strings to human-relative times ("6 hours ago").
The absolute timestamp is preserved as a native browser tooltip so full
precision is still one hover away; detail modals keep the absolute form.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/downloads.py

@@ -128,10 +128,12 @@ async def retry_download(download_id: int):
             if download.status != DownloadStatus.FAILED:
                 return jsonify({"error": "Can only retry failed downloads"}), 400
 
-            # Reset status to queued
+            # Reset status to queued and clear prior-run timestamps
             download.status = DownloadStatus.QUEUED
             download.error_type = None
             download.error_message = None
+            download.started_at = None
+            download.completed_at = None
             await session.commit()
 
             # Queue Celery task to retry the download