Add a Mobile SSO Authorization Bridge so the native app can "Sign in with
Google/Discord" without shipping any OAuth secret. It EXTENDS the existing
/auth/sso/* redirect flow (same PKCE-vs-IdP, link-only + opt-in provisioning,
TOTP gate) and terminates in the existing mobile bearer tokens — not a parallel
auth path.
- Schema: mobile_auth_sessions + mobile_auth_codes (short-lived, self-pruning;
authorization code stored hash-only, PKCE challenge is a hash by construction).
- GET /auth/mobile/sso/start: validate provider enabled + redirect_uri by EXACT
allowlist match (never prefix), seed a bridge session, reuse the SSO redirect
tagged mode:'mobile' (new redirectToIdp helper extracted from beginFlow).
- SSO callback + finishSsoTotp gain a mode:'mobile' branch: mint a single-use,
hashed, PKCE-bound code and redirect to the fixed app callback (code + echoed
state, never a token) instead of setting a cookie. 2FA keeps full parity via
the existing web TOTP form (now carrying the bridge session).
- POST /auth/mobile/sso/exchange: verify Layer-B PKCE (before burning the code),
single-use consume, then issue the SAME pair as /auth/mobile/login.
- Discovery reuses GET /auth/providers; refresh/logout reuse /auth/mobile/*.
- Rate limits: /start per-IP+provider, /exchange per-IP. Boot-time +
opportunistic prune of both tables (no cron, mirrors revoked_sessions).
- Redirect allowlist is MOBILE_AUTH_REDIRECT_URIS (default the one fixed
runicgateway://auth/callback); App Link URIs can be appended per shard later.
- Swagger regenerated; 39 tests (model single-use/gating + full controller
matrix: bad/expired/reused code, PKCE mismatch, disabled provider, redirect
allowlist, TOTP-through-bridge). Full suite green (271).
Refs docs/website/BACKEND_DESIGN.md, docs/android/PLAN.md §9 (M9).
Co-Authored-By: Claude <noreply@anthropic.com>
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