Model group delete tombstones

[grazzolini]

Milestone sequence: 8/16.

Depends on: #115.

Scope:

• Define delete_tombstone records for projects, artifacts, and entity revisions.
• Ensure offline peers cannot resurrect deleted records after reconnect.
• Route group deletes through backend services instead of raw database writes.

Acceptance:

• Project and artifact deletes are group deletes in the sync model.
• Tombstones carry author device, target identity, timestamp, and prior metadata for diagnostics.
• Delete-vs-update reconciliation rules are documented and testable.