Non-atomic notifications support

[gsmetal]

Problem

The FOR UPDATE lock on artery_model_infos is acquired inside the model's before_commit callback, meaning it is held from before_commit until the model transaction commits. Under high concurrency (30-50 Sidekiq threads writing the same model), threads queue on this lock for 7-11 seconds, causing database timeouts.

Solution

Add a per-model option non_atomic_notification that moves artery message creation from before_commit (inside the model transaction) to after_commit (in a separate short transaction). This reduces lock hold time from "remainder of the model transaction" to ~2-5ms (just the INSERT + UPDATE + COMMIT of the artery message).

# High-concurrency model — lock in a separate short transaction
artery_model source: true, non_atomic_notification: true

# Default — lock inside model transaction (fully atomic)
artery_model source: true

Trade-offs

• Default (non_atomic_notification: false): artery message creation is atomic with the model change. If message creation fails, the entire transaction rolls back. No risk of missing notifications.
• non_atomic_notification: true: artery message creation happens after the model transaction commits. If it fails, the model change is already persisted but the notification may be lost. Retry logic (up to 3 attempts) mitigates transient failures; persistent failures are reported via Artery.handle_error.