Implement automatic stream expiry and cleanup for ended streams

[Chuks-coderr]

Description

Add expiry logic so that streams past their end_time are automatically marked as completed on the next interaction, and add a pruneExpiredStreams admin entry point to reclaim storage from ended stream records.

Acceptance Criteria

• Feature is fully implemented
• Tests pass
• Lint/type checks pass
• PR references this issue number

Notes

⚠️ Do not begin coding until you have been assigned by the maintainer.
Branch as feat/401-contract-stream-expiry where N is this issue number.