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-domain/src/main/java/de/cannamanage/domain/entity/Subscription.java

@@ -0,0 +1,95 @@
+package de.cannamanage.domain.entity;
+
+import de.cannamanage.domain.enums.PlanTier;
+import de.cannamanage.domain.enums.SubscriptionStatus;
+import jakarta.persistence.*;
+
+import java.time.Instant;
+import java.util.UUID;
+
+@Entity
+@Table(name = "subscriptions")
+public class Subscription extends AbstractTenantEntity {
+
+    @Column(name = "club_id", nullable = false, unique = true)
+    private UUID clubId;
+
+    @Column(name = "stripe_customer_id", nullable = false)
+    private String stripeCustomerId;
+
+    @Column(name = "stripe_subscription_id")
+    private String stripeSubscriptionId;
+
+    @Enumerated(EnumType.STRING)
+    @Column(name = "plan_tier", nullable = false, length = 20)
+    private PlanTier planTier = PlanTier.TRIAL;
+
+    @Column(name = "member_limit", nullable = false)
+    private Integer memberLimit = 500;
+
+    @Column(name = "trial_ends_at")
+    private Instant trialEndsAt;
+
+    @Column(name = "current_period_start")
+    private Instant currentPeriodStart;
+
+    @Column(name = "current_period_end")
+    private Instant currentPeriodEnd;
+
+    @Enumerated(EnumType.STRING)
+    @Column(name = "status", nullable = false, length = 20)
+    private SubscriptionStatus status = SubscriptionStatus.TRIALING;
+
+    @Column(name = "canceled_at")
+    private Instant canceledAt;
+
+    @Column(name = "updated_at")
+    private Instant updatedAt;
+
+    @PrePersist
+    @Override
+    void onCreate() {
+        super.onCreate();
+        this.updatedAt = Instant.now();
+    }
+
+    @PreUpdate
+    void onUpdate() {
+        this.updatedAt = Instant.now();
+    }
+
+    // Getters and Setters
+
+    public UUID getClubId() { return clubId; }
+    public void setClubId(UUID clubId) { this.clubId = clubId; }
+
+    public String getStripeCustomerId() { return stripeCustomerId; }
+    public void setStripeCustomerId(String stripeCustomerId) { this.stripeCustomerId = stripeCustomerId; }
+
+    public String getStripeSubscriptionId() { return stripeSubscriptionId; }
+    public void setStripeSubscriptionId(String stripeSubscriptionId) { this.stripeSubscriptionId = stripeSubscriptionId; }
+
+    public PlanTier getPlanTier() { return planTier; }
+    public void setPlanTier(PlanTier planTier) { this.planTier = planTier; }
+
+    public Integer getMemberLimit() { return memberLimit; }
+    public void setMemberLimit(Integer memberLimit) { this.memberLimit = memberLimit; }
+
+    public Instant getTrialEndsAt() { return trialEndsAt; }
+    public void setTrialEndsAt(Instant trialEndsAt) { this.trialEndsAt = trialEndsAt; }
+
+    public Instant getCurrentPeriodStart() { return currentPeriodStart; }
+    public void setCurrentPeriodStart(Instant currentPeriodStart) { this.currentPeriodStart = currentPeriodStart; }
+
+    public Instant getCurrentPeriodEnd() { return currentPeriodEnd; }
+    public void setCurrentPeriodEnd(Instant currentPeriodEnd) { this.currentPeriodEnd = currentPeriodEnd; }
+
+    public SubscriptionStatus getStatus() { return status; }
+    public void setStatus(SubscriptionStatus status) { this.status = status; }
+
+    public Instant getCanceledAt() { return canceledAt; }
+    public void setCanceledAt(Instant canceledAt) { this.canceledAt = canceledAt; }
+
+    public Instant getUpdatedAt() { return updatedAt; }
+    public void setUpdatedAt(Instant updatedAt) { this.updatedAt = updatedAt; }
+}

cannamanage-domain/src/main/java/de/cannamanage/domain/enums/PlanTier.java

@@ -0,0 +1,8 @@
+package de.cannamanage.domain.enums;
+
+public enum PlanTier {
+    TRIAL,
+    STARTER,
+    PRO,
+    ENTERPRISE
+}

cannamanage-domain/src/main/java/de/cannamanage/domain/enums/SubscriptionStatus.java

@@ -0,0 +1,9 @@
+package de.cannamanage.domain.enums;
+
+public enum SubscriptionStatus {
+    TRIALING,
+    ACTIVE,
+    PAST_DUE,
+    CANCELED,
+    UNPAID
+}