Write a SpacetimeDB backend module in Rust that defines a user table and a stats table, with a reducer that counts total users and active users without collecting into a Vec.

TABLES
- user
  - Struct: User
  - Fields:
    - id: u64 (primary key, auto_inc)
    - name: String
    - active: bool

- user_stats
  - Struct: UserStats
  - Fields:
    - key: String (primary key)
    - count: u64

REDUCERS
- compute_user_counts(ctx)
  - Count total users by iterating with .count() (do not collect into a Vec)
  - Count active users by iterating with .filter(|u| u.active).count()
  - Insert a UserStats row with key="total" and the total count
  - Insert a UserStats row with key="active" and the active count
