Fix missing RowsAffected checks in cookie update methods

Address PR review comments: capture Exec result and check
RowsAffected() == 0 to return ErrResourceNotFound in
Cookie.Update, CookieCategory.Update, and CookieCategory.UpdateRank.
Also update coredata and relay contributor docs accordingly.

Signed-off-by: Émile Ré <emile@getprobo.com>

contrib/claude/coredata.md

@@ -129,7 +129,13 @@ assets, err := pgx.CollectRows(rows, pgx.RowToAddrOfStructByName[Asset])
 *a = assets
 
 // Update / Delete — no RETURNING
-_, err := conn.Exec(ctx, q, args)
+result, err := conn.Exec(ctx, q, args)
+if err != nil {
+    return err
+}
+if result.RowsAffected() == 0 {
+    return ErrResourceNotFound
+}
 ```
 
 ## Sentinel errors