Add and apply a models.rs convention for data-model structs

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.

DEFINITION_OF_DONE.md

@@ -14,6 +14,13 @@ A change is done when all of the following hold, not just when it compiles.
 - Within `db`, one file per domain concern (`feeds.rs`, `articles.rs`,
   `reading_events.rs`, `affinities.rs`) — a new table gets its own file,
   not a growing `queries.rs`.
+- Data-model structs (domain models, query row types, DTOs/views) live in
+  a crate's `models.rs` rather than the file that produces or consumes
+  them, whenever a struct is used outside the function that builds it —
+  matching the existing `crates/core/src/models.rs` pattern. This doesn't
+  apply to structs that are inherently local to one file/component (e.g.
+  a Dioxus component's `Props`/`Styles`/context struct, or a helper
+  struct scoped to a single function's internals).
 
 ## DRY, but not premature
 

crates/db/src/articles.rs

@@ -1,3 +1,4 @@
+use crate::models::RankedArticleRow;
 use crate::schema;
 use crate::Db;
 use anyhow::Result;
@@ -7,17 +8,6 @@ use diesel_async::RunQueryDsl;
 use feedsignal_core::Article;
 use uuid::Uuid;
 
-/// One row of [`Db::list_ranked_articles`].
-pub struct RankedArticleRow {
-    pub id: String,
-    pub feed_id: String,
-    pub title: String,
-    pub url: String,
-    pub summary: String,
-    pub topics: Vec<String>,
-    pub final_score: Option<f32>,
-}
-
 impl Db {
     pub async fn insert_article(&self, article: &Article) -> Result<()> {
         use schema::articles::dsl;

crates/db/src/lib.rs

@@ -1,7 +1,10 @@
 mod affinities;
 mod articles;
 mod feeds;
+mod models;
 mod reading_events;
+
+pub use models::RankedArticleRow;
 pub(crate) mod schema;
 
 use anyhow::Result;

crates/db/src/models.rs

@@ -0,0 +1,10 @@
+/// One row of [`crate::Db::list_ranked_articles`].
+pub struct RankedArticleRow {
+    pub id: String,
+    pub feed_id: String,
+    pub title: String,
+    pub url: String,
+    pub summary: String,
+    pub topics: Vec<String>,
+    pub final_score: Option<f32>,
+}