Geo: Implement LFS object replication for organization migration

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Problem

As part of organization migration from Legacy Cell to Protocell, the Geo team needs to implement special handling for LFS object replication. The lfs_objects table is now marked as cell-local and won't be replicated directly, requiring custom logic to ensure LFS objects are properly available on the target cell.

Background

The overall LFS migration strategy involves:

1. ✅ Copy oid from lfs_objects to lfs_object_projects (groupsource code #490482 (closed))
2. ✅ Mark lfs_objects table as cell local (completed)
3. 🔄 Migrate lfs_object_projects rows and nullify lfs_object_id (Geo team - this issue)
4. 🔄 Ensure LFS objects exist on target cell and link them (Geo team - this issue)
5. ✅ Recycle unused lfs_objects on source cell (existing functionality)

Tasks for Geo Team

Task 1: Migrate lfs_object_projects without lfs_object_id during PG data replication

When migrating an organization:

• Migrate lfs_object_projects rows for the organization (as normal)
• Nullify (or otherwise exclude from replication) the lfs_object_id reference during migration (this is the special part)
• Do NOT migrate lfs_objects rows (as usual for cell-local tables)

Task 2: Implement LFS object deduplication on target cell

After migrating lfs_object_projects rows:

• For each row in lfs_object_projects WHERE lfs_object_id IS NULL
  • Upsert an lfs_objects row with the oid, returning ID (atomic to avoid race conditions)
  • Set the lfs_object_projects lfs_object_id field
  • Insert lfs_object_registry to make the rest of Geo do its thing

Also:

• Ensure there are sufficient automated (unit? QA?) tests to meet GitLab's standards
• Smoke test locally

Related

• Blocked by Add sharding key for `lfs_objects` (#490482 - closed) because lfs_object_projects.oid needs to be backfilled and maintained in order for us to do this
• Somewhat blocked by Organization migration: Replicate PostgreSQL data (&18462) or at least some kind of Postgres replication for cells in GDK POC of Geo Protocell Mode (#571916) because we need to do Task 1 on top of Postgres replication.