[PBI] Notification correlation (grouping related events)

[cjlapao]

Description

Implement and validate the correlation_id grouping mechanism that allows multiple notifications about the same underlying entity (e.g. an alert when a host goes down and a success when it recovers) to be associated with each other. Includes the REST filter and any query helpers needed to retrieve a correlated group.

User Story

As a user investigating an alert, I want to be able to see all notifications related to the same host or event in one view, so that I can understand the full timeline — when it went wrong, whether it recovered, and what actions were taken.

Acceptance Criteria

• correlation_id stored correctly: The field is persisted verbatim from the dispatcher request. No server-side lookup or validation is performed on the value.
• REST filter works: GET /notifications?correlation_id=<value> returns all of the authenticated user's notifications sharing that correlation_id, ordered by created_at ascending (oldest first, to read as a timeline).
• Index in place: A database index on (user_id, correlation_id) exists to make the filter performant.
• Nullable correctly handled: Notifications with no correlation_id are not returned when filtering by a specific value. A filter for correlation_id=null is not supported (not a required use case).
• Dispatcher test: An integration test emits two notifications with the same correlation_id (one alert, one success) and verifies both are retrieved by the correlation filter.
• Unit tests: Cover correlation_id passthrough, correct ordering in the grouped response, and the index being used in the query plan.

Definition of Done

• Code implemented following best practices.
• Unit tests written and passing.
• Code reviewed and approved.
• Merged into the main branch.
• Documentation updated (if applicable).
• Deployed to staging/production environment.

Assumptions and Constraints

• Assumption: The caller is responsible for using a meaningful, stable correlation_id — typically the record_id of the root entity (e.g. the host's UUID).
• Constraint: The system does not enforce or validate relationships between correlated notifications. Correlation is purely a grouping convention.

Dependencies

No response

Additional Notes

No response