feat: Backend support for feature health

[khvn26]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

• Auditable feature health events
• Sample feature health event provider
• Feature health event ingestion webhook with signed paths
• "Unhealthy" system tag automatically added to features with corresponding feature health events

To create a Sample feature health provider, perform the following request:

curl -x POST http://<base-api-url>/api/v1/projects/<project-id>/feature-health/providers -H 'authorization: Token <user token>' --data-raw '{"type": "Sample"}'

or use the admin interface.

Request the resulting webhook URL:

curl -x POST http://<base-api-url>/api/v1/feature-health/<webhook-hash> --data-raw '{"environment": "<your-env-name>", "status": "unhealthy", "feature": "<your-feature-name>"}'

Observe the new system tag being added to your feature of choice.

Get a list of latest feature events per feature-provider pair:

curl http://<base-api-url>/api/v1/projects/<project-id>/feature-health/events -H 'authorization: Token <user token>'

How did you test this code?

Please describe.

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
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[github-actions]

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☁️ https://app.uffizzi.com/github.com/Flagsmith/flagsmith/pull/5023

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-5023	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-5023	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5023	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-5023	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5023	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5023	Finished ✅	Results ✅

[codecov]

Codecov Report

Attention: Patch coverage is 87.05502% with 40 lines in your changes missing coverage. Please review.

Project coverage is 97.32%. Comparing base (3557e14) to head (c233acd).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
api/features/feature_health/admin.py	0.00%	14 Missing ⚠️
api/features/feature_health/models.py	78.43%	11 Missing ⚠️
api/features/feature_health/services.py	81.81%	8 Missing ⚠️
api/features/feature_health/apps.py	0.00%	4 Missing ⚠️
api/features/feature_health/views.py	94.44%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5023      +/-   ##
==========================================
- Coverage   97.40%   97.32%   -0.08%     
==========================================
  Files        1192     1210      +18     
  Lines       41654    41984     +330     
==========================================
+ Hits        40571    40862     +291     
- Misses       1083     1122      +39     

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