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/routes/webhooks.orders.fulfilled.tsx

@@ -5,6 +5,8 @@ import {
   generateAndEmailInvoice,
   isManualPaymentOrder,
 } from "../services/invoice/automations.server";
+import { isDuplicateWebhook } from "../services/webhooks/dedupe.server";
+import { runWebhookInBackground } from "../services/webhooks/background.server";
 
 /**
  * orders/fulfilled — Automation 2: when an order is fulfilled and is NOT a
@@ -16,36 +18,42 @@ export const action = async ({ request }: ActionFunctionArgs) => {
   const { shop, topic, payload, session, admin } = await authenticate.webhook(request);
   console.log(`Received ${topic} webhook for ${shop}`);
 
+  // Idempotency against Shopify retries — see webhooks/dedupe.server.ts.
+  if (await isDuplicateWebhook(request, shop, topic)) return new Response();
+
   if (!session || !admin) {
     // App was uninstalled before the webhook drained — nothing to do.
     return new Response();
   }
 
-  const settings = await db.shopSettings.findUnique({ where: { shopDomain: shop } });
-  if (!settings?.autoEmailOnFulfilledNonWireTransfer) return new Response();
-
   const orderId = payload?.id;
   if (orderId == null) return new Response();
 
-  const orderGid = `gid://shopify/Order/${String(orderId).replace(/^.*\//, "")}`;
-  if (await isManualPaymentOrder(admin, orderGid)) {
-    // Manual / wire-transfer order — handled by Automation 1, skip here.
-    return new Response();
-  }
+  const customerLocale =
+    typeof payload?.customer_locale === "string" ? payload.customer_locale : undefined;
+
+  // Respond fast; do the heavy lifting after the response (see notes in
+  // webhooks.orders.create.tsx for the rationale).
+  runWebhookInBackground(`${topic} order=${orderId} shop=${shop}`, async () => {
+    const settings = await db.shopSettings.findUnique({ where: { shopDomain: shop } });
+    if (!settings?.autoEmailOnFulfilledNonWireTransfer) return;
+
+    const orderGid = `gid://shopify/Order/${String(orderId).replace(/^.*\//, "")}`;
+    if (await isManualPaymentOrder(admin, orderGid)) {
+      // Manual / wire-transfer order — handled by Automation 1, skip here.
+      return;
+    }
 
-  try {
     const result = await generateAndEmailInvoice({
       shopDomain: shop,
       admin,
       orderId,
-      customerLocale: typeof payload?.customer_locale === "string" ? payload.customer_locale : undefined,
+      customerLocale,
     });
     if (!result.ok) {
       console.warn(`auto-email (fulfilled) failed for order ${orderId} on ${shop}: ${result.reason}`);
     }
-  } catch (err) {
-    console.error(`auto-email (fulfilled) crashed for order ${orderId} on ${shop}:`, err);
-  }
+  });
 
   return new Response();
 };