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/webhooks/background.server.ts

@@ -0,0 +1,27 @@
+/**
+ * Fire-and-forget runner for webhook side-effects.
+ *
+ * Shopify expects a 200 response within ~5 seconds, otherwise it considers
+ * the delivery failed and retries it. Heavy automation work (PDF render,
+ * Shopify Files upload, SMTP send) routinely exceeded that budget, which
+ * caused duplicate invoice emails before we added the dedupe table.
+ *
+ * Returning the response immediately and letting the work finish in the
+ * background keeps Shopify happy. Combined with the dedupe table this is
+ * defence-in-depth: dedupe ensures *correctness* even if a retry sneaks
+ * through, while async processing makes retries unlikely in the first
+ * place.
+ *
+ * Errors are caught and logged \u2014 they cannot reach a dispatcher because
+ * the HTTP response is already gone.
+ */
+export function runWebhookInBackground(
+  description: string,
+  work: () => Promise<unknown>,
+): void {
+  // `void` so we don't accidentally `await` the floating promise; the
+  // node event loop keeps the task alive until it settles.
+  void work().catch((err) => {
+    console.error(`background webhook task '${description}' failed:`, err);
+  });
+}