Resolve a bare source slug to its canonical org-scoped home

Translation aid for clients holding a bare source slug. Returns the org-scoped tuple { sourceId, sourceSlug, orgSlug } so callers can rewrite to the canonical /v1/orgs/:orgSlug/sources/:sourceSlug path before the bare API path stops resolving slugs (#698 final piece).

Returns the oldest match by (createdAt, id) so repeated calls land on the same row. Carries Sunset: Sun, 01 Nov 2026 00:00:00 GMT on success.