fix: Flagsmith Environment Webhook incorrect payload values

[khvn26]

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Changes

Contributes to #6050, #5790.

In this PR, we fix incorrect webhook payloads when updating segment overrides (#6050). The root cause was that drf-writable-nested saves FeatureState before FeatureStateValue, so webhooks triggered on post_save captured stale values.

This is a step toward #5790 (centralising webhook logic), but further work remains. The webhook infrastructure still has multiple layers (webhooks/webhooks.py, features/tasks.py, audit/signals.py) that could be unified into a single, consistent system.

• Remove the post_save signal handler from FeatureState that triggered webhooks prematurely
• Dispatch FLAG_UPDATED webhooks via the feature_state_change_went_live signal, which fires after AuditLog creation when all nested objects are saved
• Handle both FeatureState and FeatureStateValue audit logs, since value-only changes also need to trigger webhooks
• Refactor signal semantics: FeatureState is now the sender (matching the signal name), with AuditLog as a supplemental kwarg

How did you test this code?

Added an integration test reproducing #6050, and various unit tests supporting the fix.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 3, 2026 1:41pm
flagsmith-frontend-preview	Ignored	Preview	Feb 3, 2026 1:41pm
flagsmith-frontend-staging	Ignored	Preview	Feb 3, 2026 1:41pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6646	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6646	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6646	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6646	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6646	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6646	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.20%. Comparing base (cdd3386) to head (a1d423f).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6646      +/-   ##
==========================================
+ Coverage   98.18%   98.20%   +0.01%     
==========================================
  Files        1294     1298       +4     
  Lines       46994    47108     +114     
==========================================
+ Hits        46141    46262     +121     
+ Misses        853      846       -7     

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[emyller]

Looks solid! I have comments about the implementation, and non-blocking suggestions in tests.

[emyller]

LGTM