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/cli/archivebox_extract.py

@@ -22,7 +22,7 @@ ORCHESTRATOR = None
 
 @enforce_types
 def extract(archiveresult_id: str) -> Generator['ArchiveResult', None, None]:
-    archiveresult = ArchiveResult.objects.get(Q(id=archiveresult_id) | Q(abid=archiveresult_id))
+    archiveresult = ArchiveResult.objects.get(id=archiveresult_id)
     if not archiveresult:
         raise Exception(f'ArchiveResult {archiveresult_id} not found')