-Implemented YouTube timestamp correction and added the admin Upkeeping section.

What changed:
Added shared YouTube publish-date scraping helpers in 
[includes/db.php](/Users/tyemeclifford/Documents/GH/mediaplayer/includes/db.php).
New YouTube imports now set videos.created_at to the original YouTube 
publish date when available.
Manual Add/Edit saves now also try to correct timestamps for 
YouTube-backed videos.
Added Admin Dashboard → Upkeeping → Correct YouTube timestamps in 
[admin/index.php](/Users/tyemeclifford/Documents/GH/mediaplayer/admin/index.php), 
with batch size control for backfilling existing records.
Renamed the admin table column from Added to Date, since it may now 
reflect original publish date.
Documented the behavior in 
[README.md](/Users/tyemeclifford/Documents/GH/mediaplayer/README.md).

admin/add.php

@@ -119,6 +119,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
                         $db->prepare("UPDATE videos SET thumbnail=? WHERE id=?")->execute([$thumbnail, $video_id]);
                     }
                 }
+                media_apply_youtube_timestamp($video_id);
 
                 header('Location: edit.php?id=' . $video_id . '&saved=1');
                 exit;