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-api/src/main/resources/db/migration/V7__stripe_subscriptions.sql

@@ -0,0 +1,21 @@
+-- V7: Stripe subscription management
+CREATE TABLE IF NOT EXISTS subscriptions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    club_id UUID NOT NULL REFERENCES clubs(id),
+    stripe_customer_id VARCHAR(255) NOT NULL,
+    stripe_subscription_id VARCHAR(255),
+    plan_tier VARCHAR(20) NOT NULL DEFAULT 'TRIAL',
+    member_limit INTEGER NOT NULL DEFAULT 500,
+    trial_ends_at TIMESTAMP WITH TIME ZONE,
+    current_period_start TIMESTAMP WITH TIME ZONE,
+    current_period_end TIMESTAMP WITH TIME ZONE,
+    status VARCHAR(20) NOT NULL DEFAULT 'TRIALING',
+    canceled_at TIMESTAMP WITH TIME ZONE,
+    tenant_id UUID NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+CREATE UNIQUE INDEX idx_subscriptions_club ON subscriptions(club_id);
+CREATE INDEX idx_subscriptions_stripe_customer ON subscriptions(stripe_customer_id);
+CREATE INDEX idx_subscriptions_status ON subscriptions(status);