[Jetstream] Compaction + orphan file cleanup

[dudu-theman]

Part of #138. Design doc: docs/design_docs/2026-07-13_bluesky_ingestion_jetstream.md

Periodic Iceberg compaction and unused/orphan file deletion driven by an AWS EventBridge rule, to keep merge-on-read query costs down.

Cost: EventBridge is essentially $0.00; Athena is required to run the compaction at $5.00/TB.

[coderabbitai]

🔗 Related PRs

#118 - Design Doc: Bluesky Backfill App [open]

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