Add the interests table and module (first-class interests, step 1)

Standing interests get a table of their own plus article_interests, the
per-run top-3 matches, and a pure rates() that derives each interest's
Beta-smoothed weight from current ratings the way feed affinity does.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K9PrjtUS16PAQve8D4bHgc

src/db.rs

@@ -1689,6 +1689,10 @@ mod tests {
             .execute(&pool)
             .await
             .unwrap();
+        sqlx::raw_sql(include_str!("../migrations/0013_interests.sql"))
+            .execute(&pool)
+            .await
+            .unwrap();
 
         let rows = sqlx::query(
             "SELECT article_id, issue_date, kind, source, label, value, event_at, user_id
@@ -1724,6 +1728,8 @@ mod tests {
             "interest_embeddings",
             "article_assessments",
             "candidate_runs",
+            "interests",
+            "article_interests",
             "users",
             "sessions",
             "config_changes",