Fix scheduled_at datetime format mismatch

datetime-local sends ISO format with T separator; normalize to space on save
and use SQLite datetime() in dispatcher query to handle both formats.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/dispatch-social.php

@@ -25,7 +25,7 @@ $stmt = $db->prepare("
     JOIN projects     pr ON pr.id = t.project_id AND pr.is_active = 1
     WHERE t.status = 'pending'
       AND p.status IN ('pending', 'partial')
-      AND p.scheduled_at <= ?
+      AND datetime(p.scheduled_at) <= datetime(?)
     ORDER BY p.scheduled_at ASC, t.id ASC
 ");
 $stmt->execute([$now]);