fix: Enforce seat limit self hosted

[matthewelwell]

Changes

Enforces seat limits for self-hosted deployments.

Required changes:

• API should prevent users joining organisations when organisation is already at or over seat limit
• FE should show information on the number of seats available / being used in the users settings page
• FE should gracefully handle an invite failing due to there not being enough seats available

Contributes to flagsmith/flagsmith-private#105 and https://github.com/Flagsmith/infrastructure/issues/267

How did you test this code?

Ran the FE locally to verify the FE changes. Added / updated unit tests to verify API changes.

[vercel]

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 19, 2026 10:19pm
flagsmith-frontend-preview	Ignored	Preview	Feb 19, 2026 10:19pm
flagsmith-frontend-staging	Ignored	Preview	Feb 19, 2026 10:19pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (f644ab2) to head (92b0856).
⚠️ Report is 47 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6663      +/-   ##
==========================================
+ Coverage   98.20%   98.23%   +0.02%     
==========================================
  Files        1298     1312      +14     
  Lines       47108    48503    +1395     
==========================================
+ Hits        46262    47646    +1384     
- Misses        846      857      +11     

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6663	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-6663	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6663	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6663	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6663	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6663	Finished ✅	Results ✅