Write a SpacetimeDB backend module in Rust that defines a public anonymous view over a table.

TABLE
- announcement (public)
  - Struct: Announcement
  - Fields:
    - id: u64 (primary key, auto-increment)
    - message: String
    - active: bool (btree index)

VIEW
- active_announcements (AnonymousViewContext, public)
  - Returns: Vec<Announcement>
  - Body: return all rows where active == true using the btree index
