fix: make invoice payment activation idempotent (#18)

Closes #2

## Summary
- make `markInvoicePaid` idempotent for already-paid invoices and reject invalid terminal transitions
- add admin actor metadata and audit-log writes for `mark-paid`, including replayed no-op calls
- add focused DB tests for first activation, replay safety, and invalid transition handling
- document the current payment system, including invoice creation, manual activation, quota reset, and current limitations

## 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

Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #18

apps/web/src/main.ts

@@ -5,6 +5,7 @@ import {
 } from "node:http";
 import { loadConfig } from "@nproxy/config";
 import {
+  BillingError,
   createPrismaAccountStore,
   createPrismaAuthStore,
   createPrismaBillingStore,
@@ -211,7 +212,13 @@ const server = createServer(async (request, response) => {
         }
 
         const invoiceId = decodeURIComponent(invoiceMarkPaidMatch[1] ?? "");
-        const invoice = await billingStore.markInvoicePaid({ invoiceId });
+        const invoice = await billingStore.markInvoicePaid({
+          invoiceId,
+          actor: {
+            type: "web_admin",
+            ref: authenticatedSession.user.id,
+          },
+        });
         sendJson(response, 200, {
           invoice: serializeBillingInvoice(invoice),
         });
@@ -672,6 +679,23 @@ function handleRequestError(
     return;
   }
 
+  if (error instanceof BillingError) {
+    const statusCode =
+      error.code === "invoice_not_found"
+        ? 404
+        : error.code === "invoice_transition_not_allowed"
+          ? 409
+          : 400;
+
+    sendJson(response, statusCode, {
+      error: {
+        code: error.code,
+        message: error.message,
+      },
+    });
+    return;
+  }
+
   if (error instanceof GenerationRequestError) {
     const statusCode =
       error.code === "missing_active_subscription"