Full-text search module: keyword search over any record, no model needed

Part of #249.

Keyword and full-text search over content and module records, distinct from the AI semantic module (which needs an embedding backend). Every system needs a plain "find the record that contains these words" without standing up a model.

Three unrelated systems that reuse it: a CMS (search articles), a CRM (find a contact by any field), a knowledge base (find a doc). Primitive.

Contract:
- Postgres full-text search over registered record types, ranked, tenant-scoped, RBAC-filtered.
- A module opts a record type in and picks the searchable fields; the index maintained on write.
- One query endpoint across types, paged.

Done when: a module ships with tests (index on write, query, ranking, tenant and permission filtering), and a doc showing content and a second record type searched together. Note the existing SearchText backfill work (#167, #168) is the content-side of this and should fold in.