feat(billing): add subscription and Stripe billing router

- Add stripeCustomerId column to users table
- Create Stripe client initialization (web/src/server/stripe.ts)
- Add billing service with getOrCreateCustomer, checkout/portal sessions,
  subscription management, invoice listing, and webhook event handling
- Create billing tRPC router with getSubscription, createCheckoutSession,
  createPortalSession, cancelSubscription, reactivateSubscription, listInvoices
- Add raw webhook endpoint at /api/stripe/webhook with signature verification
- Define Valibot schemas for all billing procedure inputs
- Wire billing router into root tRPC router
- Update schema tests for new column/index counts
- Write unit tests for billing service and router

web/src/server/api/routers/user.test.ts

@@ -30,6 +30,7 @@ const mockUpdateMemberRole = vi.mocked(updateMemberRole);
 type User = {
   id: string; email: string; name: string | null; image: string | null;
   role: "user" | "family_admin" | "family_member" | "support"; emailVerified: Date | null; deletedAt: Date | null;
+  stripeCustomerId: string | null;
   createdAt: Date; updatedAt: Date;
 };
 type Ctx = { db: object; user: User | null; apiKey: string | null };
@@ -97,6 +98,7 @@ function createCaller(user: User | null) {
 const baseUser: User = {
   id: "user-1", email: "a@b.com", name: "Test", image: null,
   role: "user", emailVerified: null, deletedAt: null,
+  stripeCustomerId: null,
   createdAt: new Date(), updatedAt: new Date(),
 };