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/db.ts

@@ -15,7 +15,7 @@ const poolConfig: PoolConfig = {
 
 function mockRowFromInsert(sql: string, params: unknown[]): RaceRow {
   const match = sql.match(/INSERT INTO races\s*\(([^)]+)\)\s*VALUES/i);
-  const now = new Date().toISOString();
+  const now = new Date();
   if (!match) {
     return {
       id: String(params[0] ?? ""),
@@ -108,7 +108,7 @@ function createMockPool(): Pool {
       if (!existing) {
         return emptyResult();
       }
-      const updated = { ...existing, updated_at: new Date().toISOString() };
+      const updated = { ...existing, updated_at: new Date() };
       store.set(id, updated);
       return {
         rows: [updated as unknown as T],