feat(billing): P1c Stripe dep + server client + vault-backed config

Pin stripe@22.1.1 (exact). lib/stripe/client.ts: per-request getStripe
with the pinned apiVersion (server-only, never global/client). Helpers
in lib/stripe/config.ts (plain server module — they take the admin
client so they must NOT be 'use server' exports): store/read the secret
key + webhook signing secret in the AES-256-GCM secrets vault (door
pattern), getStripeConfig. lib/actions/stripe.ts (actions only):
getStripeSettings (status only, never returns secrets) + saveStripeSettings
(write-only secrets, integrations.config upsert). Zod stripeSettingsSchema.

lib/actions/index.ts

@@ -65,3 +65,6 @@ export * from './door'
 
 // Presence & attendance (check-in / check-out / hosting)
 export * from './presence'
+
+// Stripe recurring dues (product spine Phase 1)
+export * from './stripe'