fix: Feature.type field is unbounded

[khvn26]

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• I have filled in the "Changes" section below.
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Changes

Closes #6886

In this PR, we constrain the Feature.type field to only accept valid choices (STANDARD, MULTIVARIATE), fixing a bug where arbitrary strings (e.g. "boolean") could be written and then break Pydantic validation in EnvironmentCompressed.

• Re-export FeatureType from flagsmith_schemas.types (single source of truth) and derive FEATURE_TYPE_CHOICES from it.
• Add choices=FEATURE_TYPE_CHOICES to the Feature.type model field.
• Make type read-only in CreateFeatureSerializer — it is already derived automatically by MultivariateFeatureOption lifecycle hooks.
• Add a data migration to fix existing invalid values: features with multivariate options get MULTIVARIATE, all others get STANDARD.

How did you test this code?

• Migration test: creates features with invalid type values ("boolean", "flag"), applies migration, asserts they are corrected.
• View test: verifies that type is ignored when provided in a create request and defaults to STANDARD.

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Mar 9, 2026 4:52pm
flagsmith-frontend-preview	Ignored	Preview	Mar 9, 2026 4:52pm
flagsmith-frontend-staging	Ignored	Preview	Mar 9, 2026 4:52pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6888	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6888	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6888	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6888	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6888	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6888	Finished ✅	Results ✅

[github-actions]

Playwright Test Results (oss - depot-ubuntu-latest-16)

 10 passed

Details

 10 tests across 7 suites
 46.3 seconds
 d23d3ab
 🔄 Run: #15175 (attempt 1)

[github-actions]

Playwright Test Results (oss - depot-ubuntu-latest-arm-16)

 10 passed

Details

 10 tests across 7 suites
 54.2 seconds
 d23d3ab
 🔄 Run: #15175 (attempt 1)

[github-actions]

Playwright Test Results (private-cloud - depot-ubuntu-latest-16)

 1 passed

Details

 1 test across 1 suite
 1 minute, 2 seconds
 d23d3ab
 🔄 Run: #15175 (attempt 1)

[github-actions]

Playwright Test Results (private-cloud - depot-ubuntu-latest-arm-16)

 2 passed

Details

 2 tests across 2 suites
 1 minute, 1 second
 d23d3ab
 🔄 Run: #15175 (attempt 1)

[emyller]

🚢it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.34%. Comparing base (c5953c2) to head (d23d3ab).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6888   +/-   ##
=======================================
  Coverage   98.33%   98.34%           
=======================================
  Files        1334     1335    +1     
  Lines       49661    49707   +46     
=======================================
+ Hits        48836    48882   +46     
  Misses        825      825           

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