fix: Use CustomUserAdmin to fix user creation bug (#1726)

### Summary

Fixed the bug where users created via the web GUI cannot login.

### Root Cause

The issue was in `archivebox/core/admin.py` which imported and
registered Django's default `UserAdmin` instead of the custom
`CustomUserAdmin` class. This bypassed all custom admin logic.
Additionally, `CustomUserAdmin` was modifying `fieldsets` without
explicitly preserving `add_fieldsets`, which could cause Django to not
properly handle the user creation form.

### Changes

- Updated `admin.py` to import and register `CustomUserAdmin`
- Explicitly set `add_fieldsets` in `CustomUserAdmin` to preserve
Django's default user creation behavior
- Added explanatory comments

### Testing

To verify the fix:
1. Start ArchiveBox web server
2. Navigate to the admin user creation page (`/admin/auth/user/add/`)
3. Create a new user with staff and superuser permissions
4. Log out and attempt to log in with the new user's credentials
5. Login should now succeed

Fixes #1707

Generated with [Claude Code](https://claude.ai/code)

archivebox/core/admin.py

@@ -7,11 +7,11 @@ from archivebox.core.models import Snapshot, ArchiveResult, Tag
 from archivebox.core.admin_tags import TagAdmin
 from archivebox.core.admin_snapshots import SnapshotAdmin
 from archivebox.core.admin_archiveresults import ArchiveResultAdmin
-from archivebox.core.admin_users import UserAdmin
+from archivebox.core.admin_users import CustomUserAdmin
 
 
 def register_admin(admin_site):
-    admin_site.register(get_user_model(), UserAdmin)
+    admin_site.register(get_user_model(), CustomUserAdmin)
     admin_site.register(ArchiveResult, ArchiveResultAdmin)
     admin_site.register(Snapshot, SnapshotAdmin)
     admin_site.register(Tag, TagAdmin)

archivebox/core/admin_users.py

@@ -10,6 +10,12 @@ class CustomUserAdmin(UserAdmin):
     sort_fields = ['id', 'email', 'username', 'is_superuser', 'last_login', 'date_joined']
     list_display = ['username', 'id', 'email', 'is_superuser', 'last_login', 'date_joined']
     readonly_fields = ('snapshot_set', 'archiveresult_set', 'tag_set', 'apitoken_set', 'outboundwebhook_set')
+
+    # Preserve Django's default user creation form and fieldsets
+    # This ensures passwords are properly hashed and permissions are set correctly
+    add_fieldsets = UserAdmin.add_fieldsets
+
+    # Extend fieldsets for change form only (not user creation)
     fieldsets = [*UserAdmin.fieldsets, ('Data', {'fields': readonly_fields})]
 
     @admin.display(description='Snapshots')