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.

app/services/invoice/i18n.ts

@@ -167,11 +167,19 @@ const en: InvoiceStrings = {
   paidStamp: "PAID",
 };
 
+// Locale → invoice language. We only render in German (`de`) when the
+// caller is explicitly German-speaking (de, de-AT, de-DE, de_CH, …).
+// Everything else (it, fr, es, en, …) falls back to English so that
+// non-German-speaking customers don't receive a German invoice. Callers
+// that have a per-shop default fall back to it via
+// `pickLanguage(customerLocale ?? settings.defaultLanguage)`, which is why
+// `null`/`undefined` still maps to `de` (the legacy default for the
+// Austrian shops this app was built for).
 export function pickLanguage(input: string | null | undefined): InvoiceLanguage {
   if (!input) return "de";
   const v = input.toLowerCase();
-  if (v.startsWith("en")) return "en";
-  return "de";
+  if (v.startsWith("de")) return "de";
+  return "en";
 }
 
 export function getStrings(language: InvoiceLanguage): InvoiceStrings {