refactor(compute): filter logs on subservice column, not source attribute

[johnstonmatt]

The compute logs query relied on log_attributes['source'] to identify the stream, but the writer now publishes this as a top-level subservice column rather than a log_attributes key, so filtering switches to that column directly.

• Filter and project on the top-level subservice column instead of log_attributes['source'] in computeLogsQuery
• Update unit and integration tests to assert on subservice and confirm log_attributes['source'] is no longer referenced
• Update SIDE_EFFECTS.md to describe the new filter columns