feat(billing): add weekly_pro subscription plan

Adds a 2.99 EUR/week plan with a 3-day free trial alongside the existing
monthly and yearly subscriptions.

Backend: weekly_pro joins the supported subscription products, the
available product list and the Discord sales label. No schema change --
weekly Pro grants the same 100 credits per calendar month as monthly Pro,
so no column is needed to tell the two apart.

Paywall: weekly and yearly are the two prominent cards, monthly is a
selectable row below them. Weekly is preselected. Cards only render when
their RevenueCat package exists, and the selection falls back to a
visible plan so the CTA can never buy a product that is not loaded.

Trial eligibility: checkTrialOrIntroductoryPriceEligibility now gates the
trial copy. Apple grants one intro offer per subscription group, so with
two trial products a second free-trial promise would otherwise be shown
to users who get charged immediately. Anything but a clear ELIGIBLE is
treated as no trial, as the RevenueCat SDK recommends.

Analytics: trial_started previously fired on every subscription purchase,
including monthly which never had a trial. It now fires only for products
that actually carry one. paywall_viewed distinguishes trial_enabled from
trial_eligible and reports selected_plan.

Tests: 9 new cases covering the entitlement path, credits, renewal period
and trial allowance.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

__tests__/services/mockBackendService.test.ts

@@ -241,6 +241,76 @@ describe('mockBackendService billing simulation', () => {
     expect(response.billing.entitlement.renewsAt).toBe('2026-04-30T00:00:00.000Z');
   });
 
+  it('grants pro from a weekly_pro purchase and renews after 7 days', async () => {
+    const response = await mockBackendService.simulatePurchase({
+      userId: 'test-user-weekly-purchase',
+      idempotencyKey: 'weekly-1',
+      productId: 'weekly_pro',
+    });
+
+    expect(response.appliedProduct).toBe('weekly_pro');
+    expect(response.billing.entitlement.plan).toBe('pro');
+    expect(response.billing.credits.monthlyAllowance).toBe(100);
+
+    // Das Wochenabo verlaengert sich nach 7 Tagen, nicht nach 30 wie die anderen Plaene.
+    const renewsInDays = (new Date(response.billing.entitlement.renewsAt as string).getTime() - Date.now())
+      / (24 * 60 * 60 * 1000);
+    expect(renewsInDays).toBeCloseTo(7, 3);
+  });
+
+  it('offers weekly_pro as a purchasable product', async () => {
+    const response = await mockBackendService.simulatePurchase({
+      userId: 'test-user-weekly-products',
+      idempotencyKey: 'weekly-products-1',
+      productId: 'topup_small',
+    });
+
+    expect(response.billing.availableProducts).toContain('weekly_pro');
+  });
+
+  it('syncs pro entitlement from a weekly_pro RevenueCat subscription', async () => {
+    // Regressionsschutz: fehlt weekly_pro im unterstuetzten Produkt-Set, verwirft der
+    // lokale Sync das aktive Entitlement und der Kaeufer bleibt auf 'free'.
+    const response = await mockBackendService.syncRevenueCatState({
+      userId: 'test-user-rc-weekly',
+      customerInfo: {
+        entitlements: {
+          active: {
+            pro: {
+              productIdentifier: 'weekly_pro',
+              expirationDate: '2026-04-30T00:00:00.000Z',
+            },
+          },
+        },
+        nonSubscriptions: {},
+      },
+    });
+
+    expect(response.billing.entitlement.plan).toBe('pro');
+    expect(response.billing.entitlement.status).toBe('active');
+  });
+
+  it('limits a weekly_pro trial to trial credits', async () => {
+    const response = await mockBackendService.syncRevenueCatState({
+      userId: 'test-user-rc-weekly-trial',
+      customerInfo: {
+        entitlements: {
+          active: {
+            pro: {
+              productIdentifier: 'weekly_pro',
+              expirationDate: '2026-04-30T00:00:00.000Z',
+              periodType: 'TRIAL',
+            },
+          },
+        },
+        nonSubscriptions: {},
+      },
+    });
+
+    expect(response.billing.entitlement.plan).toBe('pro');
+    expect(response.billing.credits.monthlyAllowance).toBe(30);
+  });
+
   it('limits RevenueCat trial entitlement to trial credits', async () => {
     const response = await mockBackendService.syncRevenueCatState({
       userId: 'test-user-rc-trial',