feat: Add 5-day post-mortem reminders and critical-incidents notifications

[NicolasLafitteMM]

Summary

This PR implements automated post-mortem reminders and notifications for the FireFighter incident management system:

• Post-mortem creation announcements: When a PM is created for P1-P3 production incidents, an announcement is automatically sent to #critical-incidents
• 5-day overdue reminders: Automated reminders sent 5 days after an incident reaches MITIGATED status, posted to both the incident channel and #critical-incidents (for eligible incidents)
• Periodic execution: Reminders run twice daily at 10 AM and 3 PM (Paris time)
• Testing tools: Management commands to test the feature without waiting 5 days

Changes

Core Features

• New Slack messages (slack_messages.py):

  • SlackMessageIncidentPostMortemCreatedAnnouncement: Announcement for PM creation in #critical-incidents
  • SlackMessagePostMortemReminder5Days: Reminder message for incident channel
  • SlackMessagePostMortemReminder5DaysAnnouncement: Reminder announcement for #critical-incidents

• New Celery task (send_postmortem_reminders.py):

  • Periodic task to check incidents mitigated 5+ days ago
  • Sends reminders to incident channel and #critical-incidents (when applicable)
  • Prevents duplicate reminders using Message tracking

• New rule function (slack/rules.py):

  • should_publish_pm_in_general_channel(): Determines if PM announcements should be sent to #critical-incidents (P1-P3 production incidents only)

Database Changes

• Incident model: Added mitigated_at timestamp field to track when incidents reach MITIGATED status
• Celery Beat: Created periodic task with CrontabSchedule for 10 AM and 3 PM executions

Code Quality

• Refactored postmortem_created_handler: Reduced complexity by extracting helper functions:
  • _update_mitigated_at_timestamp()
  • _create_confluence_postmortem()
  • _create_jira_postmortem()
  • _publish_postmortem_announcement()

Testing Tools

• Management command backdate_incident_mitigated: Backdate incident mitigated_at timestamp for testing
• Management command test_postmortem_reminders: Execute reminder task manually with optional --list-only mode
• Documentation: Comprehensive testing guide at docs/contributing/testing-postmortem-reminders.md

Test Plan

• Run migrations successfully
• Backdate test incident by 6 days
• Execute test_postmortem_reminders command
• Verify messages sent to incident channel
• Verify messages sent to #critical-incidents for eligible incidents
• Verify duplicate prevention (Message table tracking)
• Verify status filter includes both MITIGATED and POST_MORTEM
• All type checks pass (mypy)
• All linting passes (ruff)

Files Changed

• 12 files modified
• +893 insertions, -34 deletions

New Files

• docs/contributing/testing-postmortem-reminders.md
• src/firefighter/incidents/management/commands/backdate_incident_mitigated.py
• src/firefighter/incidents/management/commands/test_postmortem_reminders.py
• src/firefighter/incidents/migrations/0030_add_mitigated_at_field.py
• src/firefighter/slack/migrations/0009_add_postmortem_reminder_periodic_task.py
• src/firefighter/slack/tasks/send_postmortem_reminders.py

Modified Files

• src/firefighter/incidents/models/incident.py
• src/firefighter/jira_app/signals/postmortem_created.py
• src/firefighter/slack/messages/slack_messages.py
• src/firefighter/slack/rules.py

[codecov-commenter]

Codecov Report

❌ Patch coverage is 28.03738% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.26%. Comparing base (8ee053b) to head (869dea8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...firefighter/jira_app/signals/postmortem_created.py	20.00%	40 Missing ⚠️
src/firefighter/slack/messages/slack_messages.py	33.33%	36 Missing ⚠️
src/firefighter/slack/rules.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
- Coverage   67.49%   67.26%   -0.24%     
==========================================
  Files         213      213              
  Lines        9938    10027      +89     
  Branches     1086     1097      +11     
==========================================
+ Hits         6708     6745      +37     
- Misses       2941     2993      +52     
  Partials      289      289              

Flag	Coverage Δ
unittests	67.26% <28.03%> (-0.24%)	⬇️

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