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/src/services/billing.ts

@@ -0,0 +1,50 @@
+import { useMutation, useQuery } from "@tanstack/react-query"
+
+import { apiClient } from "@/lib/api-client"
+
+export interface SubscriptionData {
+  planTier: string
+  status: string
+  memberLimit: number
+  trialEndsAt: string | null
+  currentPeriodStart: string | null
+  currentPeriodEnd: string | null
+  canceledAt: string | null
+  hasStripeSubscription: boolean
+}
+
+export function useSubscriptionQuery() {
+  return useQuery({
+    queryKey: ["billing", "subscription"],
+    queryFn: () => apiClient<SubscriptionData>("/billing/subscription"),
+  })
+}
+
+export function useCreateCheckoutMutation() {
+  return useMutation({
+    mutationFn: async (planTier: string) => {
+      const data = await apiClient<{ url: string }>("/billing/checkout", {
+        method: "POST",
+        body: { planTier },
+      })
+      return data
+    },
+    onSuccess: (data) => {
+      window.location.href = data.url
+    },
+  })
+}
+
+export function useCreatePortalMutation() {
+  return useMutation({
+    mutationFn: async () => {
+      const data = await apiClient<{ url: string }>("/billing/portal", {
+        method: "POST",
+      })
+      return data
+    },
+    onSuccess: (data) => {
+      window.location.href = data.url
+    },
+  })
+}