docs(issues): add affine android OIDC sign-up failure issue

start tracking service issues on the docs/issues directory

docs/issues/affine/250420_android_oidc.md

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+# Android application OIDC issue
+
+## Status
+- Processing
+
+## Date
+- 2026-04-20
+
+## version
+- affine server: 0.26.3 (self-hosted)
+- affine application: 0.26.3 (Android)
+- IdP: Authelia:4.39.15
+
+## Problem
+- Affine android app cannot authenticate via OIDC
+  - IdP authentication succeeds, but the app does not establish a session
+  - The app remains on the "Sign In" screen
+
+## Reason
+- Affine uses callback deep link `affine://authentication`
+- For self-hosted instances the deep link carries a 'server' parameter pointing to the correct origin, but android never read it.
+- [Issue #12819: No SSO on Android](https://github.com/toeverything/AFFiNE/issues/12819)
+- [PR #14809](https://github.com/toeverything/AFFiNE/pull/14809)
+
+## Timeline
+- 2025-06-14: Issue #12819
+- 2026-04-08: PR #14809
+- 2026-04-09: Canary branch merge
+- 2026-04-15: Fork, cherry-pick
+
+## Solution
+- Wait for stable release which contains the merge above
+- When the stable version releases, then verify after update