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>

server/lib/discord.js

@@ -9,6 +9,7 @@ const PURCHASE_EVENT_TYPES = new Set([
 ]);
 
 const PLAN_NAMES_BY_PRODUCT = {
+  weekly_pro: 'Pro (wöchentlich)',
   monthly_pro: 'Pro (monatlich)',
   yearly_pro: 'Pro (jährlich)',
   topup_small: 'Top-up Small (30 Credits)',