[4.2] User API handlers: list_users, create_user, get_user

[lomartins]

Phase 4.2 — User API handlers (Medium)

Fill in the handler bodies in backend/src/api/mod.rs. Claude provides the function signatures.

Handlers to implement

// GET /api/v1/users
async fn list_users(State(pool): State<PgPool>) -> impl IntoResponse

// POST /api/v1/users
async fn create_user(
    State(pool): State<PgPool>,
    Json(req): Json<CreateUserRequest>,
) -> impl IntoResponse

// GET /api/v1/users/:id
async fn get_user(
    State(pool): State<PgPool>,
    Path(id): Path<i64>,
) -> impl IntoResponse

Each handler should:

1. Call the corresponding UserService method
2. Return Json(ApiResponse { data: result }) on success
3. Return an appropriate error via AppError on failure

Prerequisite: Phase 3 models + Phase 4.1 UserService must be complete.

Done when: curl -X POST localhost:3000/api/v1/users ... returns a User JSON object.