feat: add maintenance window lifecycle workflow triggers

[aditya-786]

Summary

Adds a maintenance workflow trigger so workflows can run when a maintenance window changes. This is the lifecycle-events feature requested in #6532, useful for notifying stakeholders (Slack, PagerDuty, email, etc.) when maintenance is scheduled or removed.

A workflow subscribes to the actions it cares about:

triggers:
  - type: maintenance
    events:
      - created
      - updated
      - deleted

The window is exposed to the workflow through a maintenance context, so steps can template values like {{ maintenance.name }}, {{ maintenance.cel_query }}, {{ maintenance.start_time }} and {{ maintenance.end_time }}.

How it works

This mirrors the existing incident trigger end to end:

• MaintenanceWindowDto is the event object passed to the workflow (in keep/api/models/db/maintenance_window.py).
• WorkflowManager.insert_maintenance enqueues every enabled workflow subscribed to the fired action, reusing a small get_maintenance_events helper for the trigger matching (mirrors insert_incident).
• The maintenance CRUD routes call it after commit, wrapped in a try/except so a workflow failure can never break the maintenance API (mirrors IncidentBl.send_workflow_event).
• WorkflowScheduler and ContextManager route the new DTO through the same paths already used for alerts and incidents (metrics trigger_type, triggered_by label, event_type, and the maintenance context key).

No schema migration and no new background job are required.

Scope

This PR covers the created / updated / deleted actions, which are one-shot hooks on the CRUD routes. The time-based started / ended transitions from the issue are intentionally left for a follow-up, since they need a persisted notified-state and a periodic check and are better reviewed on their own.

Tests

tests/test_maintenance_workflow_trigger.py:

• get_maintenance_events filters by trigger type and flattens the subscribed actions.
• insert_maintenance enqueues a matching workflow with triggered_by="maintenance:created" and the window as the event.
• insert_maintenance skips a workflow when the fired action is not one it subscribed to.

Relates to #6532

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