feat: add renewal invoice sweep (#20)

Refs #9

## Summary
- add a worker-side renewal invoice sweep that creates one invoice 72 hours before subscription expiry
- expire elapsed pending invoices automatically and email users when an automatic renewal invoice is created
- stop auto-recreating invoices for the same paid cycle once any invoice already exists for that cycle
- document the current renewal-invoice and pending-invoice expiry behavior

## Testing
- built `infra/docker/web.Dockerfile`
- ran `pnpm --filter @nproxy/db test` inside the built container
- verified `@nproxy/db build` and `@nproxy/web build` during the image build
- built `infra/docker/worker.Dockerfile`

Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #20

packages/providers/src/payments.ts

@@ -17,8 +17,18 @@ export interface CreatedProviderInvoice {
   expiresAt: Date;
 }
 
+export type ProviderInvoiceStatus = "pending" | "paid" | "expired" | "canceled";
+
+export interface ProviderInvoiceStatusRecord {
+  providerInvoiceId: string;
+  status: ProviderInvoiceStatus;
+  paidAt?: Date;
+  expiresAt?: Date;
+}
+
 export interface PaymentProviderAdapter {
   createInvoice(input: PaymentInvoiceDraft): Promise<CreatedProviderInvoice>;
+  getInvoiceStatus(providerInvoiceId: string): Promise<ProviderInvoiceStatusRecord>;
 }
 
 export function createPaymentProviderAdapter(config: {
@@ -37,6 +47,13 @@ export function createPaymentProviderAdapter(config: {
           expiresAt: new Date(Date.now() + 30 * 60 * 1000),
         };
       },
+
+      async getInvoiceStatus(providerInvoiceId) {
+        return {
+          providerInvoiceId,
+          status: "pending",
+        };
+      },
     };
   }
 
@@ -51,5 +68,12 @@ export function createPaymentProviderAdapter(config: {
         expiresAt: new Date(Date.now() + 30 * 60 * 1000),
       };
     },
+
+    async getInvoiceStatus(providerInvoiceId) {
+      return {
+        providerInvoiceId,
+        status: "pending",
+      };
+    },
   };
 }