test: add unit tests for CRUD router

[Thomas-Sedhom]

Replaces #133 (closed by stale bot)

Added a full unit test suite for the buildCrudRouter utility in crud.ts. The tests cover all five HTTP methods the router exposes and use a mock DB layer so there's no real database dependency.
What's covered

GET /

• Returns paginated results and correctly caps limit at 100 even when a higher value is passed
• Enriches user items with an empty roles array when typeName is users

POST /

• Runs schema validation, custom validation, transformRequest, and transformResponse in the right order and with the right data
• Returns 400 with field-level details when schema validation fails
• Returns 400 with custom error messages when custom validation fails
• Returns 409 on duplicate key constraint violations (pg error code 23505)

GET /:id

• Returns a single item passed through transformResponse
• Returns 404 when the record doesn't exist

PUT /:id

• Updates the record and returns it transformed
• Returns 409 on unique constraint violation

DELETE /:id

• Deletes the record and returns { status: 'deleted' }