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/migrations/20260515083638_add_processed_webhook/migration.sql

@@ -0,0 +1,13 @@
+-- Idempotency table for inbound Shopify webhooks. We insert a row keyed on
+-- the X-Shopify-Webhook-Id header at the start of webhook processing; a
+-- duplicate insert (P2002) means Shopify retried a delivery we've already
+-- seen, so we short-circuit and return 200 without doing the work twice.
+CREATE TABLE "ProcessedWebhook" (
+    "webhookId"  TEXT NOT NULL PRIMARY KEY,
+    "topic"      TEXT NOT NULL,
+    "shopDomain" TEXT NOT NULL,
+    "receivedAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX "ProcessedWebhook_shopDomain_topic_idx"
+    ON "ProcessedWebhook"("shopDomain", "topic");