fix(observability,webhooks,i18n): timestamped logs, dedupe webhook retries, default non-de locales to English

- New custom server.js (replaces react-router-serve): ISO timestamps on
  all console.* output and on access logs, and skip successful /healthz
  polls so real traffic stays visible.
- New ProcessedWebhook table + dedupe helper keyed on
  X-Shopify-Webhook-Id; stops Shopify retries from triggering a second
  invoice email when the original delivery exceeded the 5s ack timeout.
- orders/create + orders/fulfilled now respond 200 immediately and run
  the PDF/email work in the background so we stay under that timeout.
- pickLanguage(): non-German locales (it, fr, es, ...) now default to
  English instead of falling back to German. Empty/unknown still maps to
  'de' so the per-shop defaultLanguage chain keeps working.
- Tests for pickLanguage and dedupe via node --test + tsx.

prisma/schema.prisma

@@ -182,6 +182,17 @@ model EmailLog {
   @@index([shopDomain, invoiceId])
 }
 
+// Idempotency table for inbound Shopify webhooks. See
+// `app/services/webhooks/dedupe.server.ts` for details.
+model ProcessedWebhook {
+  webhookId   String   @id
+  topic       String
+  shopDomain  String
+  receivedAt  DateTime @default(now())
+
+  @@index([shopDomain, topic])
+}
+
 // Per-shop logo bytes cache. Avoids fetching the logo from Shopify Files on
 // every PDF render.
 model LogoCache {