feat(billing): TD-13 webhook idempotency + Stripe Customer Portal + doc cleanup

- Table stripe_webhook_events + helpers isEventProcessed/markEventProcessed
- POST /stripe/customer-portal (auth + stripe_customer_id check)
- ARCHITECTURE-backend.md: suppression POST /plans/upgrade (duplication doc)
- TD-13 fermé dans TECH_DEBT-backend.md
- Tests: 261 → 278 verts (+17)

src/lib/__tests__/createBillingPortalSession.test.ts

@@ -0,0 +1,69 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const portalCreateMock = vi.fn();
+
+vi.mock("stripe", () => ({
+  default: vi.fn(() => ({
+    billingPortal: {
+      sessions: {
+        create: portalCreateMock,
+      },
+    },
+  })),
+}));
+
+import { createBillingPortalSession } from "../stripe";
+
+describe("createBillingPortalSession", () => {
+  beforeEach(() => {
+    portalCreateMock.mockReset();
+  });
+
+  it("retourne l'URL de la billing portal session", async () => {
+    portalCreateMock.mockResolvedValue({
+      url: "https://billing.stripe.com/p/session/abc123",
+    });
+
+    const result = await createBillingPortalSession({
+      customerId: "cus_abc",
+      returnUrl: "https://expria.app/dashboard",
+    });
+
+    expect(result.url).toBe("https://billing.stripe.com/p/session/abc123");
+    expect(portalCreateMock).toHaveBeenCalledWith({
+      customer: "cus_abc",
+      return_url: "https://expria.app/dashboard",
+    });
+  });
+
+  it("throw si customerId vide", async () => {
+    await expect(
+      createBillingPortalSession({
+        customerId: "",
+        returnUrl: "https://expria.app/dashboard",
+      }),
+    ).rejects.toThrow("customerId requis");
+    expect(portalCreateMock).not.toHaveBeenCalled();
+  });
+
+  it("throw si returnUrl vide", async () => {
+    await expect(
+      createBillingPortalSession({
+        customerId: "cus_abc",
+        returnUrl: "",
+      }),
+    ).rejects.toThrow("returnUrl requis");
+    expect(portalCreateMock).not.toHaveBeenCalled();
+  });
+
+  it("throw si Stripe ne retourne pas d'URL", async () => {
+    portalCreateMock.mockResolvedValue({ url: null });
+
+    await expect(
+      createBillingPortalSession({
+        customerId: "cus_abc",
+        returnUrl: "https://expria.app/dashboard",
+      }),
+    ).rejects.toThrow("URL de billing portal");
+  });
+});