docs: add POST support for self-hosted JWT callback endpoint

[thesandlord]

docs: add POST support for self-hosted JWT callback endpoint

Summary

Updates the self-hosted authentication docs to reflect that the JWT callback endpoint (/api/fern-docs/auth/jwt/callback) now supports POST with application/x-www-form-urlencoded body, in addition to the existing GET query parameter flow. This documents the feature added in fern-platform#7059.

Changes:

• Updated the "How it works" and "Building your login server" sections to describe both GET and POST methods
• Wrapped the existing Node.js example in <Tabs> with a new POST tab showing an auto-submitting HTML form pattern
• Added a changelog entry for 2026-02-12

Updates since last revision

• Fixed grammar: "in a application/x-www-form-urlencoded" → "in an application/x-www-form-urlencoded" (line 45 of auth doc)

Review & Testing Checklist for Human

• Verify the changelog link /learn/docs/self-hosted/authentication#basic-token-verification resolves to the correct page and anchor in the preview
• Check <Tabs> rendering in the preview — confirm both GET and POST tabs display and switch correctly
• XSS in POST example: The template literal injects redirectUri and token directly into HTML without escaping. Acceptable for a documentation example, but worth a conscious decision since users may copy-paste this into production code

Suggested test plan: Open the preview link, navigate to the self-hosted authentication page, and verify the tabbed code examples render correctly. Check the changelog page for the new entry.

Notes

• Vale CI check passed
• The POST tab duplicates the base64url/createFernJWT helper code from the GET tab to keep each example self-contained
• Local testing confirmed all sections render correctly (tabs, changelog entry, formatting)

Link to Devin run | Requested by @thesandlord

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[github-actions]

🌿 Preview your docs: https://fern-preview-24b4c0bd-46fa-4eae-a7aa-338cc628112e.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/2/12
• learn/docs/self-hosted/authentication

[thesandlord]

remove "self-hosted", this callback endpoint works for all envs

[devin-ai-integration]

Removed "self-hosted" from both the changelog heading and body. Pushed in 133d641.

[thesandlord]

add a link to the auth overview page instead of self hosted specific page

[devin-ai-integration]

Updated to link to the auth overview page instead. Pushed in 19b6301.