Promote APP_LINKS.md from a deferred design note into an implementation spec
matching the website `feat/mobile-app-links` and android `feat/app-links`
branches: the server-side `/.well-known/assetlinks.json` route + `mobile_app_links_enabled`
toggle + additive redirect-allowlist entry, and the app-side `autoVerify`
intent-filter driven by a build-time `appLinkHost` (a single multi-tenant APK
cannot autoVerify open-ended shard domains, so App Links are a white-label /
first-party build opt-in; the custom scheme stays the permanent fallback).
Update PLAN.md §9 (M9 follow-up) and the §14 open item, and the redirect-URI
allowlist section of website/BACKEND_DESIGN.md.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NgyHnrNa8WwG3doxvxjuCr
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