Show issue owner in alert notifications

## Current behavior

Alert notifications (Slack and email) do not include the issue owner. Users must click through to the alert link and navigate to the issue list to find owner assignments.

## Gap

Teams using alert rules tied to specific projects (e.g. mobile, onboarding) need to quickly route or action alerts. Without owner information in the notification itself, every alert requires manual navigation to identify ownership, adding friction and slowing response time.

## Proposed behavior

- Include the assigned owner(s) directly in Slack and email alert notification messages
- No additional navigation required to identify issue ownership

## Related

- #84073 — enriching alerts with issue context (covers metrics like event frequency and affected users, but not ownership)

Action taken on behalf of David Quintas.

Read related issue #84073 for existing alert-context work, then trace the Slack and email alert notification entry points and where issue owners are available. Done means both message formats include assigned owner(s) without requiring navigation; verify the behavior in the relevant notification tests.

Written by the indexing model from the issue text.