Add PUT /users/:id endpoint for updating users

[Pagnarith]

Summary

• Implemented PUT /users/:id endpoint to update existing users
• Added updateUser() helper to db/store.js for data access
• Validates required fields (name, email) and returns 400 if missing
• Returns 404 for non-existent users, 200 with updated user on success

Test Plan

✅ All 9 tests passing:

• PUT /users/:id updates an existing user
• PUT /users/:id returns 404 for a user that does not exist
• PUT /users/:id with a missing field returns 400
• All existing endpoints continue to pass

Implementation Notes

• Follows existing patterns from GET and POST endpoints
• Consistent error handling and status codes
• Code review found no runtime-correctness issues

🤖 Generated with Claude Code