Migrate desk metadata to assets #106

Migrate desks from desks metadata on level zones into the asset table. The migration runs in a single transaction and is safe to re-run.

• Collect: authorities that have config.org_zone → zone tree → level branches → desks metadata
• Prepare: lenient field mapping (map_id/identifier fall back to id; security becomes an array)
• Transaction: create or reuse the _DESKS_ category and type per authority; upsert assets on (type, zone_id, map_id)
• Remap desk bookings: asset_id + asset_ids → Asset#id (also booking_instances overrides); report misses
• Merge {"desk": {"use_assets": true}} into each org zone's settings metadata
• Print a summary; add an opt-in dry_run=true (rolls back)
• migrate:desk_assets sam task + README
• Spec suite for init (./test, docker compose, CI job) covering mapping, misses, re-runs and dry run