Apply same composite-FK pattern as 001/002 for migration 003:
- resources: add UNIQUE (id, space_id) as FK target.
- resource_dependencies: denormalize space_id, composite FKs on both endpoints
(enforces both ends of a dep live in the same space at the DB layer).
- resource_credentials: denormalize space_id, composite FK to resources.
- source_docs.resource_id: NOT NULL (tenancy anchor); resource_id was already
absent from the update FIELDS list so docs cannot move resources.
Repos derive space_id from the resource on addDependency/addCredential so callers
can't fake cross-tenant assignment. 3 regression tests added.
