Write a SpacetimeDB backend module in Rust that models a minimal ECS and computes next positions.

TABLES
- entity
  - Struct: Entity
  - Fields:
    - id: u64 (primary key, auto_inc)

- position
  - Struct: Position
  - Fields:
    - entity_id: i32 (primary key) — NOT auto_inc, this is a foreign key to entity
    - x: i32
    - y: i32

- velocity
  - Struct: Velocity
  - Fields:
    - entity_id: i32 (primary key) — NOT auto_inc, this is a foreign key to entity
    - vx: i32
    - vy: i32

- next_position
  - Struct: NextPosition
  - Fields:
    - entity_id: i32 (primary key) — NOT auto_inc, this is a foreign key to entity
    - x: i32
    - y: i32

REDUCERS
- seed: insert the two entities with positions/velocities (entity IDs are auto-assigned):
  - Entity 1: pos=(0,0), vel=(1,0)
  - Entity 2: pos=(10,0), vel=(-2,3)
- step: for each position, find velocity by entity_id; compute (x+vx, y+vy) and upsert into next_position by entity_id
