Remove ABID system and KVTag model - use UUIDv7 IDs exclusively

This commit completes the simplification of the ID system by:

- Removing the ABID (ArchiveBox ID) system entirely
- Removing the base_models/abid.py file
- Removing KVTag model in favor of the existing Tag model in core/models.py
- Simplifying all models to use standard UUIDv7 primary keys
- Removing ABID-related admin functionality
- Cleaning up commented-out ABID code from views and statemachines
- Deleting migration files for ABID field removal (no longer needed)

All models now use simple UUIDv7 ids via `id = models.UUIDField(primary_key=True, default=uuid7)`

Note: Old migrations containing ABID references are preserved for database
migration history compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

archivebox/api/v1_api.py

@@ -70,7 +70,7 @@ class NinjaAPIWithIOCapture(NinjaAPI):
 
         response['X-ArchiveBox-Auth-Method'] = getattr(request, '_api_auth_method', None) or 'None'
         response['X-ArchiveBox-Auth-Expires'] = token_expiry
-        response['X-ArchiveBox-Auth-Token-Id'] = api_token.abid if api_token else 'None'
+        response['X-ArchiveBox-Auth-Token-Id'] = str(api_token.id) if api_token else 'None'
         response['X-ArchiveBox-Auth-User-Id'] = request.user.pk if request.user.pk else 'None'
         response['X-ArchiveBox-Auth-User-Username'] = request.user.username if request.user.pk else 'None'