docs: add comprehensive Stripe billing guide

Covers architecture, configuration steps, user flows, API routes,
webhooks, pricing, testing with Stripe CLI, production checklist,
and troubleshooting.

memento-note/components/usage-meter.tsx

@@ -188,7 +188,7 @@ export function UsageMeter({ className }: UsageMeterProps) {
                       onClick={(e) => { e.stopPropagation(); router.push('/settings/billing'); }}
                       className="w-full mt-2 py-2 bg-brand-accent/10 hover:bg-brand-accent text-brand-accent hover:text-white text-[9px] font-bold uppercase tracking-widest rounded-xl transition-all duration-300 border border-brand-accent/20"
                     >
-                      {t('usageMeter.upgradePricing') || 'Passer à Pro'}
+                      {t('usageMeter.upgradePricing')}
                     </button>
                   )}
                 </div>