feat(sprint-6): Phase 3 — Stripe integration (SEPA + PayPal + Card)

- V7 migration: subscriptions table with plan tiers
- Subscription entity + PlanTier/SubscriptionStatus enums
- StripeService: customer creation, checkout, portal, webhook handling
- SubscriptionController: /api/v1/billing endpoints
- Webhook handler: invoice.paid, payment_failed, subscription.deleted/updated
- Plan enforcement: member limit interceptor, trial expiry check
- Frontend: /settings/billing page (plan card, usage, upgrade, portal link)
- Trial expired banner on all admin pages
- React Query hooks (useSubscriptionQuery, checkout/portal mutations)
- Stripe Java SDK 28.2.0
- Full i18n (de/en) for billing namespace

cannamanage-frontend/messages/en.json

@@ -370,5 +370,34 @@
     "deleteButton": "Permanently delete account",
     "deleteConfirm": "Are you sure? This action cannot be undone.",
     "deleteSuccess": "Your account has been deleted. You will now be logged out."
+  },
+  "billing": {
+    "title": "Plan & Billing",
+    "currentPlan": "Current Plan",
+    "trial": "Free Trial",
+    "starter": "Starter",
+    "pro": "Pro",
+    "enterprise": "Enterprise",
+    "trialEnds": "Trial ends on {date}",
+    "trialExpired": "Your trial has expired. Choose a plan to continue.",
+    "trialDaysLeft": "{days} days remaining",
+    "memberLimit": "Member limit",
+    "membersUsed": "{used} / {limit} members",
+    "price": "{price}/month",
+    "nextBilling": "Next billing date",
+    "upgrade": "Upgrade plan",
+    "manageBilling": "Manage payment details",
+    "invoices": "Invoices",
+    "noInvoices": "No invoices yet.",
+    "starterDesc": "For small clubs up to 30 members",
+    "proDesc": "For growing clubs up to 100 members",
+    "enterpriseDesc": "For large clubs — unlimited members",
+    "starterPrice": "€19",
+    "proPrice": "€49",
+    "enterprisePrice": "On request",
+    "selectPlan": "Select plan",
+    "active": "Active",
+    "pastDue": "Past due",
+    "canceled": "Canceled"
   }
 }