feat(db): add field-definition registry repository

Implements create_field_definition, field_definition_by_key, and
list_field_definitions in db::fields, with TDD integration tests
covering text, term, and authority field type round-trips.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/db/src/lib.rs

@@ -3,6 +3,7 @@
 pub mod audit;
 pub mod authority;
 pub mod catalog;
+pub mod fields;
 pub mod vocab;
 
 use sqlx::postgres::{PgPool, PgPoolOptions};