fix: address review feedback — URLField→CharField, safe reverse migration

- Change SocialMediaLink.url and NavigationMenuItem.link_url from
  URLField to CharField(max_length=500) to support internal paths
  like /feed/ that fail URLField validation
- Replace destructive reverse_seed (deleted ALL rows) with
  RunPython.noop to prevent data loss on rollback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/core/models.py

@@ -85,10 +85,11 @@ class NavigationMenuItem(Orderable):
         related_name="+",
         help_text="Link to an internal page. If unpublished, the link is hidden automatically.",
     )
-    link_url = models.URLField(
+    link_url = models.CharField(
+        max_length=500,
         blank=True,
         default="",
-        help_text="External URL (used only when no page is selected).",
+        help_text="URL or path (used only when no page is selected).",
     )
     link_title = models.CharField(
         max_length=100,
@@ -144,7 +145,7 @@ class SocialMediaLink(Orderable):
         max_length=30,
         choices=SOCIAL_ICON_CHOICES,
     )
-    url = models.URLField()
+    url = models.CharField(max_length=500, help_text="URL or path (e.g. https://twitter.com/… or /feed/).")
     label = models.CharField(
         max_length=100,
         blank=True,