feat: enhance Letzshop order import with EAN matching and stats

- Add historical order import with pagination support
- Add customer_locale, shipping_country_iso, billing_country_iso columns
- Add gtin/gtin_type columns to Product table for EAN matching
- Fix order stats to count all orders server-side (not just visible page)
- Add GraphQL introspection script with tracking workaround tests
- Enrich inventory units with EAN, MPN, SKU, product name
- Add LetzshopOrderStats schema for proper status counts

Migrations:
- a9a86cef6cca: Add locale and country fields to letzshop_orders
- cb88bc9b5f86: Add gtin columns to products table

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/letzshop.py

@@ -94,6 +94,13 @@ class LetzshopOrder(Base, TimestampMixin):
     )  # Store as string to preserve format
     currency = Column(String(10), default="EUR")
 
+    # Customer preferences (for invoicing)
+    customer_locale = Column(String(10), nullable=True)  # en, fr, de
+
+    # Shipping/billing country
+    shipping_country_iso = Column(String(5), nullable=True)  # LU, DE, FR, etc.
+    billing_country_iso = Column(String(5), nullable=True)
+
     # Raw data storage (for debugging/auditing)
     raw_order_data = Column(JSON, nullable=True)