Your question is Backfill While Preserving Real Time. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You need to backfill terabytes of historical data into a pipeline that is already processing live events. The key challenge is keeping downstream tables correct and queryable without pausing real-time ingestion or introducing duplicates, stale overwrites, or long freshness regressions.