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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Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-preview	Ready	Preview, Comment	Feb 20, 2026 4:36pm
flagsmith-frontend-staging	Ready	Preview, Comment	Feb 20, 2026 4:36pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 20, 2026 4:36pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (f644ab2) to head (e87994c).
⚠️ Report is 48 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-api-test:pr-6663	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e: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 ✅

[Zaimwa9]

I'd like to confirm that the difference i'm seeing on the error message display is not due to our local setups somehow

[Zaimwa9]

Also I'm realising this will not work with invitation from link.

I am not sure whether it's on purpose as there is a disabled flag verify_seats_limit_for_invite_links used in account-store.controller.acceptInvite.
If we try to join an organisation via link while max seats have been reached the error from the link request gets overridden by the one from users/join/id that will return the No Invite matches the given query:

  acceptInvite: (id) => {
    store.saving()
    API.setInvite('')
    API.setInviteType('')
    return data
      .post(`${Project.api}users/join/link/${id}/`)
      .catch((error) => {
        console.log('error-ajax', error)
        if (
          Utils.getFlagsmithHasFeature('verify_seats_limit_for_invite_links') &&
          error.status === 400
        ) {
          API.ajaxHandler(store, error)
          return // Here it should throw too otherwise below res.id is undefined and crash the app
        }
        return data.post(`${Project.api}users/join/${id}/`)
      })

Then I don't know if this is the way self-hosted are supposed to invite their members

[matthewelwell]

As per discussion, and manual testing, I think this is now resolved in e87994c.