feat(cohorts): Amplitude cohort sync endpoints and sync keys

[gagantrivedi]

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Contributes to Flagsmith/flagsmith-private#260.

WIP: receiving side of Amplitude's list-based cohort sync contract — per-environment sync keys plus create-list/add/remove endpoints feeding the membership ledger.

How did you test this code?

Please describe.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Aug 14, 2026 9:03am
flagsmith-frontend-preview	Ignored	Preview	Aug 14, 2026 9:03am
flagsmith-frontend-staging	Ignored	Preview	Aug 14, 2026 9:03am

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 672f4a66-2279-4e5d-90ae-674bcbb1ed8b

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[codecov]

Codecov Report

❌ Patch coverage is 99.69040% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.75%. Comparing base (dd04ba6) to head (bd48a94).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
api/cohorts/authentication.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #8290    +/-   ##
========================================
  Coverage   98.75%   98.75%            
========================================
  Files        1569     1577     +8     
  Lines       62828    63196   +368     
========================================
+ Hits        62044    62411   +367     
- Misses        784      785     +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.