Document in the DoD that data-model structs (domain models, query row
types, DTOs/views) belong in a crate's models.rs rather than the file
that produces/consumes them, once used outside that function -
matching the existing crates/core/src/models.rs pattern. Component-
local structs (Props, Styles, context) are exempt.
Apply it to db: move RankedArticleRow out of articles.rs into a new
crates/db/src/models.rs.
