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/tasks/downloads.py

@@ -265,6 +265,7 @@ async def _download_source_async(source_id: int, download_id: int = None) -> dic
 
                     download.status = DownloadStatus.RUNNING
                     download.started_at = utcnow()
+                    download.completed_at = None
                 else:
                     logger.warning(f"Download {download_id} not found, creating new record")
                     download = None