fix: lock aggregation call to one per user concurrently

[JoshC8C7]

Greptile Summary

Prevented deadlocks during concurrent batch transaction aggregation by using PostgreSQL advisory locks.

• Used pg_try_advisory_xact_lock with per-user locking to serialize aggregation calls
• If lock cannot be acquired, aggregation is skipped gracefully since it's a lazy optimization
• Lock is transaction-scoped and automatically released, preventing resource leaks
• Read queries remain functional with slightly stale data when aggregation is deferred

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is a focused concurrency fix using standard PostgreSQL advisory lock patterns. The implementation correctly uses transaction-level locks, handles lock acquisition failures gracefully, and maintains data correctness even when aggregation is skipped. The lock key is properly scoped per-user with a unique prefix to avoid collisions.
• No files require special attention

Important Files Changed

Filename	Overview
dwctl/src/db/handlers/credits.rs	Added per-user advisory lock to prevent concurrent aggregation deadlocks; implementation is clean and handles lock contention gracefully

_{Last reviewed commit: ee39dd3}

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