fix(proxy): assert the production Supabase project for customer hosts instead of allowlisting them (#2107)

The white-label guard only fired for hostnames hand-listed in
CUSTOMER_PRODUCTION_WHITE_LABEL_HOSTS. improveone.accounted.se was never added,
so when the .accounted.se domains were pinned to a feature-branch preview the
guard let it through: a customer-branded login page served from a build that
inlines the staging Supabase project, on the open internet, with no alert. The
26 August willem.accounted.se 503s were the same misrouting caught correctly,
because willem was on the list.

Inverts the model. Any customer-facing production hostname (not a *.vercel.app
preview, not localhost) must be served by the production Supabase project or
the guard trips. Adding a new white-label host no longer requires editing a
list in order to be protected.

Also closes two fail-open holes found alongside it. parseBackendHostname
returned null for an undefined NEXT_PUBLIC_SUPABASE_URL, so a missing project
read as "not staging" and fell through; it now trips the guard. And proxy.ts
gains an explicit env guard: today lib/supabase/middleware.ts asserts the URL
and key non-null and @supabase/ssr throws synchronously as the first statement
of updateSessionInner, which takes down every path including /login and
/robots.txt with an opaque crash rather than a deliberate 503.


Claude-Session: https://claude.ai/code/session_016ifKg6Ec67A39oxfGPU1yc

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

DECISIONS.md

@@ -1427,6 +1427,7 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-09-01] multi_user skeptic fixes: Stripe cancel EXPIRES the multi_user stripe grant instead of deleting it (grace anchor; other grants still deleted per freeze-and-retain); app-side state checks go RPC-first via SECURITY DEFINER company_multi_user_state (capability_grants RLS hides team rows from non-team users, byrå clients would misread as frozen); byra-kind teams get a standing team-scoped multi_user grant via backfill + teams trigger (WL-10 assumption made real; partner billing is out-of-band); PGRST202 on resolution fails OPEN (pre-migration DB has no multi_user rows: gated fallback would freeze all non-owners); /api/v1 got the same dormancy gate as MCP. RLS-level enforcement and the mid-session API fallback write-back window stay v2 follow-ups (documented, same class as pre-existing stale-preference fallback).
 [2026-09-01] Declined CodeRabbit's UpgradeNote suggestion (PR #1758 follow-up) to append the self-host connector sentence to children instead of replacing them: every caller's children is hosted subscription copy ("... kräver ett abonnemang"), so appending would show subscription wording on a self-host, the exact thing the branch exists to avoid; the "CSV/SIE import stays free" text it cited is a code comment in BankSyncNowButton, not children. Replace-on-self-host stays; a dedicated selfHosted children prop can come when a caller actually needs per-panel reassurance there.
 [2026-09-01] getConnectorConfig() rebuilds baseUrl as origin + path (userinfo/query/fragment stripped, warn-logged without the raw value): /api/connector/status echoes baseUrl to the operator and the sync/proxy URLs get paths appended, so nothing secret-shaped pasted into GNUBOK_CONNECT_URL may survive; the stripped parts were never meaningful in a base URL. The status route is also Cache-Control: no-store (key prefix + wiring layout out of shared browser caches).
+[2026-09-01] White-label backend guard inverted from an allowlist of protected hosts to an assertion that any customer-facing production host is served by the production Supabase project. This reverses the earlier explicit-allowlist decision: that model failed open for improveone.accounted.se, which was serving a byra login page wired to the staging project with no alert, because it was never added to the list.
 [2026-09-01] Anon-callable SECURITY DEFINER writes: the guard shape `IF auth.uid() IS NOT NULL AND NOT EXISTS (membership)` is unsafe on its own. The anon JWT carries no `sub` claim, so auth.uid() is NULL for role anon too and the guard short-circuits into the trusted branch. It is defense in depth behind a REVOKE FROM PUBLIC, anon, never a substitute for one. Every new SECURITY DEFINER function ships with that REVOKE; tests/pg/definer-function-grants.pg.test.ts enforces it from a sweep rather than a hand list, because hand-listing is exactly how three guarded numbering RPCs were wrongly declared safe.
 [2026-09-01] public.create_invoice_with_items(jsonb,jsonb) is revoked, not dropped. It is prod-only, uncalled and already non-functional, so removing it is cleanup rather than security, and the revoke closes the hole in full. An irreversible schema deletion against production belongs in its own reviewable migration.
 [2026-09-01] SKV connector refresh classification fixed broker-side (skeptic refutation on PR #2103, found independently by two skeptics): the broker's /oauth/token catch-all had collapsed SKV's terminal dead-refresh-token dialects (404 id_not_found, 400 invalid_grant, "Refresh Token status is expired": the DOMINANT refresh outcome, per-flow tokens live 65 min) into the generic 502, so a connector instance could never classify ordinary session expiry: raw English 500s instead of the reconnect flow, staged filing ops consumed as non-recoverable, crons retrying raw forever. The broker now re-codes those dialects (refresh grant only, never the code exchange where invalid_grant means an expired one-shot code) as 401 CONNECTOR_SKV_REFRESH_DEAD, which the instance maps to SESSION_EXPIRED; the generic 502 remains raw so a transient SKV outage still never re-arms the reconnect banner (#1155). Same pass: the data proxy now forwards WWW-Authenticate + x-skv-*/x-amzn-*/x-api-* response headers (the instance's MISSING_SCOPE classification reads them; nothing secret rides in them), and the instance's gateway-refusal guidance is connector-aware (a self-host has no SKATTEVERKET_APIGW_CLIENT_ID or Utvecklarportalen access: point at /api/connector/status + support instead).