Adds migration 0004_org_to_instance, the ScopedCollections list, the
AssertNoScopedCollectionMissed boot check, and moves EnsureAuthIndexes into
its own file.
Two ordering constraints the rename exposed, both now enforced and commented:
- 0004 must run BEFORE EnsureAuthIndexes. The index builder creates
instances.slug, which would create an empty instances collection and make
0004 refuse to rename orgs onto an existing target.
- Migrations 0001 to 0003 run BEFORE 0004 and still read and write org_id, so
they use a private legacyOrg struct rather than shared/models.