chore: delete empty features module, fix RaceRow timestamp types

- Remove dead frontend/src/features/index.ts (empty export, unused)

- RaceRow: created_at/updated_at typed as Date to match pg TIMESTAMPTZ runtime

- rowToDto: explicit toISOString() conversion instead of relying on JSON.stringify

- Mock DB: return Date objects for timestamp fields to match real pg behavior

Made-with: Cursor

backend/src/mappers/race.ts

@@ -1,4 +1,7 @@
-/** Row shape returned by PostgreSQL (snake_case). */
+/**
+ * Row shape returned by PostgreSQL (snake_case).
+ * pg returns DATE as string, NUMERIC as string, TIMESTAMPTZ as Date.
+ */
 export interface RaceRow {
   id: string;
   race_date: string;
@@ -13,8 +16,8 @@ export interface RaceRow {
   finish_time: string | null;
   finish_place: string | null;
   notes: string | null;
-  created_at: string;
-  updated_at: string | null;
+  created_at: Date;
+  updated_at: Date | null;
 }
 
 /** API shape (camelCase). */
@@ -36,6 +39,10 @@ export interface RaceDto {
   updatedAt: string | null;
 }
 
+function toISOString(value: Date | string): string {
+  return value instanceof Date ? value.toISOString() : String(value);
+}
+
 /** Convert a DB row to the API DTO (camelCase). */
 export function rowToDto(row: RaceRow): RaceDto {
   return {
@@ -52,8 +59,8 @@ export function rowToDto(row: RaceRow): RaceDto {
     finishTime: row.finish_time,
     finishPlace: row.finish_place,
     notes: row.notes,
-    createdAt: row.created_at,
-    updatedAt: row.updated_at,
+    createdAt: toISOString(row.created_at),
+    updatedAt: row.updated_at ? toISOString(row.updated_at) : null,
   };
 }