feat: add subscriptions feature capsule - M1 database schema and Stripe client (refs #55)

- Create 4 new tables: subscriptions, subscription_events, donations, tier_vehicle_selections
- Add StripeClient wrapper with createCustomer, createSubscription, cancelSubscription,
  updatePaymentMethod, createPaymentIntent, constructWebhookEvent methods
- Implement SubscriptionsRepository with full CRUD and mapRow case conversion
- Add domain types for all subscription entities
- Install stripe npm package v20.2.0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/subscriptions/domain/subscriptions.types.ts

@@ -0,0 +1,133 @@
+/**
+ * @ai-summary Type definitions for subscriptions feature
+ * @ai-context Core business types for Stripe subscription management
+ */
+
+// Subscription tier types (matches DB enum)
+export type SubscriptionTier = 'free' | 'pro' | 'enterprise';
+
+// Subscription status types (matches DB enum)
+export type SubscriptionStatus = 'active' | 'past_due' | 'canceled' | 'unpaid';
+
+// Billing cycle types (matches DB enum)
+export type BillingCycle = 'monthly' | 'yearly';
+
+// Donation status types (matches DB enum)
+export type DonationStatus = 'pending' | 'succeeded' | 'failed' | 'canceled';
+
+// Main subscription entity
+export interface Subscription {
+  id: string;
+  userId: string;
+  stripeCustomerId: string;
+  stripeSubscriptionId?: string;
+  tier: SubscriptionTier;
+  billingCycle?: BillingCycle;
+  status: SubscriptionStatus;
+  currentPeriodStart?: Date;
+  currentPeriodEnd?: Date;
+  gracePeriodEnd?: Date;
+  cancelAtPeriodEnd: boolean;
+  createdAt: Date;
+  updatedAt: Date;
+}
+
+// Subscription event entity (webhook event logging)
+export interface SubscriptionEvent {
+  id: string;
+  subscriptionId: string;
+  stripeEventId: string;
+  eventType: string;
+  payload: Record<string, any>;
+  createdAt: Date;
+}
+
+// Donation entity (one-time payments)
+export interface Donation {
+  id: string;
+  userId: string;
+  stripePaymentIntentId: string;
+  amountCents: number;
+  currency: string;
+  status: DonationStatus;
+  createdAt: Date;
+  updatedAt: Date;
+}
+
+// Tier vehicle selection entity (tracks which vehicles user selected to keep during downgrade)
+export interface TierVehicleSelection {
+  id: string;
+  userId: string;
+  vehicleId: string;
+  selectedAt: Date;
+}
+
+// Request/Response types
+
+export interface CreateSubscriptionRequest {
+  userId: string;
+  tier: SubscriptionTier;
+  billingCycle: BillingCycle;
+  paymentMethodId?: string;
+}
+
+export interface SubscriptionResponse {
+  id: string;
+  userId: string;
+  stripeCustomerId: string;
+  stripeSubscriptionId?: string;
+  tier: SubscriptionTier;
+  billingCycle?: BillingCycle;
+  status: SubscriptionStatus;
+  currentPeriodStart?: string;
+  currentPeriodEnd?: string;
+  gracePeriodEnd?: string;
+  cancelAtPeriodEnd: boolean;
+  createdAt: string;
+  updatedAt: string;
+}
+
+export interface DonationResponse {
+  id: string;
+  userId: string;
+  stripePaymentIntentId: string;
+  amountCents: number;
+  currency: string;
+  status: DonationStatus;
+  createdAt: string;
+  updatedAt: string;
+}
+
+export interface CreateDonationRequest {
+  userId: string;
+  amountCents: number;
+  currency?: string;
+}
+
+export interface CreateSubscriptionEventRequest {
+  subscriptionId: string;
+  stripeEventId: string;
+  eventType: string;
+  payload: Record<string, any>;
+}
+
+export interface CreateTierVehicleSelectionRequest {
+  userId: string;
+  vehicleId: string;
+}
+
+// Service layer types
+export interface UpdateSubscriptionData {
+  stripeSubscriptionId?: string;
+  tier?: SubscriptionTier;
+  billingCycle?: BillingCycle;
+  status?: SubscriptionStatus;
+  currentPeriodStart?: Date;
+  currentPeriodEnd?: Date;
+  gracePeriodEnd?: Date;
+  cancelAtPeriodEnd?: boolean;
+}
+
+export interface UpdateDonationData {
+  status?: DonationStatus;
+}