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

TABLE
- Announcement (public)
  - Fields:
    - Id: ulong (primary key, auto-increment)
    - Message: string
    - Active: bool (btree index)

VIEW
- ActiveAnnouncements (AnonymousViewContext, public)
  - Returns: IEnumerable<Announcement>
  - Body: return all rows where Active == true using the btree index filter
