Write a SpacetimeDB backend module in Rust 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
- event_log
  - Struct: EventLog
  - Fields:
    - id: u64 (primary key, auto_inc)
    - category: String
    - severity: u32
    - message: String
  - Index: multi-column btree index on (category, severity)

- filtered_event
  - Struct: FilteredEvent
  - Fields:
    - event_id: u64 (primary key)
    - message: String

REDUCERS
- filter_events(ctx, category: String, severity: u32)
  - Iterate all event_log rows
  - For each row where category matches AND severity matches, insert into filtered_event
