Write a SpacetimeDB backend module in C# that defines an EventLog table with a multi-column BTree index and a result table, with a reducer that filters events by both category and severity level.

TABLES
- EventLog
  - Struct: EventLog
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Category: string
    - Severity: uint
    - Message: string
  - Index: multi-column btree index on (Category, Severity)

- FilteredEvent
  - Struct: FilteredEvent
  - Fields:
    - EventId: ulong (primary key)
    - Message: string

REDUCERS
- FilterEvents(ctx, string category, uint severity)
  - Iterate all EventLog rows
  - For each row where Category matches AND Severity matches, insert into FilteredEvent
