Write a SpacetimeDB backend module in TypeScript that defines an event log 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
  - Fields:
    - id: number (u64, primary key, autoInc)
    - category: string
    - severity: number (u32)
    - message: string
  - Index: multi-column btree index on (category, severity)

- filteredEvent
  - Fields:
    - eventId: number (u64, primary key)
    - message: string

REDUCERS
- filter_events(ctx, { category: string, severity: number (u32) })
  - Iterate all eventLog rows
  - For each row where category matches AND severity matches, insert into filteredEvent
