Decode list_ranked_articles rows via a From<Row> impl

Replaces the inline .map() closure with From<Row> for
RankedArticleRow, separating "what a raw SQLite row looks like" from
the query itself.

crates/db/src/articles.rs

@@ -108,7 +108,6 @@ impl Db {
         if let Some(feed_id) = feed_id {
             query = query.filter(dsl::feed_id.eq(feed_id.to_string()));
         }
-        type Row = (String, String, String, String, String, String, Option<f32>);
         let rows: Vec<Row> = query
             .order(dsl::final_score.desc())
             .limit(limit)
@@ -123,20 +122,25 @@ impl Db {
             ))
             .load(&mut conn)
             .await?;
-        Ok(rows
-            .into_iter()
-            .map(
-                |(id, feed_id, title, url, summary, topics_json, final_score)| RankedArticleRow {
-                    id,
-                    feed_id,
-                    title,
-                    url,
-                    summary,
-                    topics: serde_json::from_str(&topics_json).unwrap_or_default(),
-                    final_score,
-                },
-            )
-            .collect())
+        Ok(rows.into_iter().map(RankedArticleRow::from).collect())
+    }
+}
+
+/// Raw shape of one `list_ranked_articles` row as loaded from SQLite —
+/// `topics` is still the JSON string column, not yet decoded.
+type Row = (String, String, String, String, String, String, Option<f32>);
+
+impl From<Row> for RankedArticleRow {
+    fn from((id, feed_id, title, url, summary, topics_json, final_score): Row) -> Self {
+        Self {
+            id,
+            feed_id,
+            title,
+            url,
+            summary,
+            topics: serde_json::from_str(&topics_json).unwrap_or_default(),
+            final_score,
+        }
     }
 }