fix(auth): resolve auth-link hosts from the brands table, drop NEXT_PUBLIC_WHITELABEL_DOMAINS (#2376)

* fix(auth): resolve auth-link hosts from the brands table, drop NEXT_PUBLIC_WHITELABEL_DOMAINS

Password reset, invite, email change and signup links now resolve the
request host against brands.domain server-side. The env var was a second
copy of that registry compiled into the browser; every new brand needed
the row, the env var, the GoTrue allowlist and a redeploy, and two
partners shipped with the env var stale, so their reset mails went out
canonical-branded to the canonical host.

- New POST /api/auth/password-reset: the login page no longer calls
  GoTrue directly, so the browser carries no domain list.
- lib/domains/trusted-app-origin.ts is async and registry-backed; it
  also trusts this deployment's own VERCEL_URL / VERCEL_BRANCH_URL so
  previews keep sending links to themselves.
- Signup shares the same resolver instead of following the raw host.
- Docs and .env.example describe the single registry; GoTrue keeps the
  redirect allowlist as backstop (hosted: *.accounted.se wildcard).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NUNB7qjua8EUaJmZgfFscx

* fix(auth): await the async origin resolver in the billing routes merged from main

PR #2370 added resolveRequestAppOrigin callers in billing/checkout and
billing/portal after this branch made the resolver async. Await them and
move their tests from the removed env var to the brands mock; update the
login source-assert test to the server-routed reset.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NUNB7qjua8EUaJmZgfFscx

* fix(auth): refuse auth links on a failed brand lookup, keep local dev hosts, correct GoTrue allowlist docs

Skeptic and CI findings on #2376, one pass:

- A failed brands lookup now throws BrandLookupFailedError (TRANSIENT_ERROR,
  503, retryable) instead of falling back to the canonical origin: a
  canonical link is the wrong-brand mail this PR removes. Password reset
  and email change answer 503 themselves; withRouteContext routes map the
  code.
- A local canonical (dev) trusts other local hosts and ports on the same
  scheme, so lane servers on 3001-3003 confirm signups on themselves.
- GoTrue matches the full redirect_to including the query and `*` stops
  at `.` and `/`: docs and decision line now prescribe
  https://*.accounted.se/auth/callback** and https://*.accounted.se/invite/**.
- The Turnstile contract test asserts the server-routed reset forwards
  the captcha token (it still asserted the removed browser call).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NUNB7qjua8EUaJmZgfFscx

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

lib/auth/oauth-flow-binding.ts

@@ -90,12 +90,17 @@ export function redactUserId(id: string | null | undefined): string {
  * never dragged to the canonical login), or the request origin itself on a
  * self-hosted deployment with no NEXT_PUBLIC_APP_URL.
  */
-export function buildLoginRedirect(request: Request, returnOrigin?: string | null): Response {
+export async function buildLoginRedirect(
+  request: Request,
+  returnOrigin?: string | null,
+): Promise<Response> {
   const current = new URL(request.url)
+  // A login bounce carries no token, so a failed brands lookup degrades to
+  // the canonical host rather than failing the callback.
   const appOrigin = returnOrigin
-    ? resolveTrustedAppOrigin(returnOrigin)
+    ? await resolveTrustedAppOrigin(returnOrigin, { onLookupFailure: 'canonical' })
     : process.env.NEXT_PUBLIC_APP_URL
-      ? resolveRequestAppOrigin(request)
+      ? await resolveRequestAppOrigin(request, { onLookupFailure: 'canonical' })
       : current.origin
   const next = `${current.pathname}${current.search}`
   const login = new URL('/login', appOrigin)
@@ -135,7 +140,7 @@ export async function requireFlowInitiator(
     return {
       ok: false,
       reason: 'no_session',
-      response: buildLoginRedirect(request, options.returnOrigin),
+      response: await buildLoginRedirect(request, options.returnOrigin),
     }
   }