refactor(Segments): Clean up after Segment Change Request work

[emyller]

Closes #5825

Changes

• Make Segment.version.default = 1 at field level instead of using a hook
• Inherit MetadataSerializerMixin from Serializer Moved to new branch
• Make Segment.version_of = null the indication for a live segment Moved to fix(Segments): Improve data model around live segments #6275.

Implementation Notes

Note

Migration 0030_add_default_to_segment_version:

• Non-blocking and safe — schema-only change
• Field remains nullable for backward compatibility
• All existing segments already have version=1 (set by migration 0025_set_default_version_on_segment)

[vercel]

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3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored	Preview		Nov 12, 2025 9:17am
flagsmith-frontend-preview	Ignored	Preview		Nov 12, 2025 9:17am
flagsmith-frontend-staging	Ignored	Preview		Nov 12, 2025 9:17am

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.02%. Comparing base (f8a3381) to head (97a3b77).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6265      +/-   ##
==========================================
- Coverage   98.02%   98.02%   -0.01%     
==========================================
  Files        1280     1280              
  Lines       45408    45401       -7     
==========================================
- Hits        44510    44503       -7     
  Misses        898      898              

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6265	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6265	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6265	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6265	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6265	Finished ✅	Results ✅

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[matthewelwell]

A couple of minor comments, but nothing that I see as a blocker if we want to get this merged ahead of the sprint closure.