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/workers/orchestrator.py

@@ -173,7 +173,7 @@ class Orchestrator:
         
                     next_obj = queue.first()
                     print()
-                    print(f'🏃‍♂️ {self}.runloop() {actor_type.__name__.ljust(20)} queue={str(queue.count()).ljust(3)} next={next_obj.abid if next_obj else "None"} {next_obj.status if next_obj else "None"} {(timezone.now() - next_obj.retry_at).total_seconds() if next_obj and next_obj.retry_at else "None"}')
+                    print(f'🏃‍♂️ {self}.runloop() {actor_type.__name__.ljust(20)} queue={str(queue.count()).ljust(3)} next={next_obj.id if next_obj else "None"} {next_obj.status if next_obj else "None"} {(timezone.now() - next_obj.retry_at).total_seconds() if next_obj and next_obj.retry_at else "None"}')
                     self.idle_count = 0
                     try:
                         existing_actors = actor_type.get_running_actors()