audit: remove duplicate Stripe integration card + tighten input validation

Deep audit of the recent change set (passes 56-60):

- Remove the generic Integrations-tab Stripe card. saveIntegration
  overwrites integrations.config wholesale, so configuring Stripe there
  clobbered the dues config (mode/prices/grace_days) that the Dues tab
  manages, and couldn't actually set up dues. Stripe is now configured in
  one place (the Dues tab). PayPal/Zeffy/Venmo sync cards unchanged.
- deleteDuesPaymentMethod validates platform against DUES_LINK_PLATFORMS
  instead of passing a raw client string into the enum column.
- markNotificationsRead uses a strict uuid regex for the id filter.

PGRST201 class characterized + confirmed contained: PostgREST treats a
2-FK table as an ambiguous junction only when its PK covers both FKs; the
only such table now is space_member_custom_roles (a legitimate M2M).
Guarded by integration/auth-embed.test.ts.

lib/actions/notifications.ts

@@ -77,7 +77,9 @@ export async function markNotificationsRead(
   if (input.ids && input.ids.length > 0) {
     // Defend the uuid column against a malformed client id (a non-uuid would
     // error the cast). Only well-formed ids pass; an empty result is a no-op.
-    const ids = input.ids.filter(id => /^[0-9a-f-]{36}$/i.test(id))
+    const ids = input.ids.filter(id =>
+      /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i.test(id),
+    )
     if (ids.length === 0) return { ok: true }
     q = q.in('id', ids)
   }