feat: integer cents money handling, order page fixes, and vendor filter persistence

Money Handling Architecture:
- Store all monetary values as integer cents (€105.91 = 10591)
- Add app/utils/money.py with Money class and conversion helpers
- Add static/shared/js/money.js for frontend formatting
- Update all database models to use _cents columns (Product, Order, etc.)
- Update CSV processor to convert prices to cents on import
- Add Alembic migration for Float to Integer conversion
- Create .architecture-rules/money.yaml with 7 validation rules
- Add docs/architecture/money-handling.md documentation

Order Details Page Fixes:
- Fix customer name showing 'undefined undefined' - use flat field names
- Fix vendor info empty - add vendor_name/vendor_code to OrderDetailResponse
- Fix shipping address using wrong nested object structure
- Enrich order detail API response with vendor info

Vendor Filter Persistence Fixes:
- Fix orders.js: restoreSavedVendor now sets selectedVendor and filters
- Fix orders.js: init() only loads orders if no saved vendor to restore
- Fix marketplace-letzshop.js: restoreSavedVendor calls selectVendor()
- Fix marketplace-letzshop.js: clearVendorSelection clears TomSelect dropdown
- Align vendor selector placeholder text between pages

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

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

app/services/cart_service.py

@@ -6,6 +6,9 @@ This module provides:
 - Session-based cart management
 - Cart item operations (add, update, remove)
 - Cart total calculations
+
+All monetary calculations use integer cents internally for precision.
+See docs/architecture/money-handling.md for details.
 """
 
 import logging
@@ -19,6 +22,7 @@ from app.exceptions import (
     InvalidCartQuantityException,
     ProductNotFoundException,
 )
+from app.utils.money import cents_to_euros
 from models.database.cart import CartItem
 from models.database.product import Product
 
@@ -62,21 +66,23 @@ class CartService:
             extra={"item_count": len(cart_items)},
         )
 
-        # Build response
+        # Build response - calculate totals in cents, return euros
         items = []
-        subtotal = 0.0
+        subtotal_cents = 0
 
         for cart_item in cart_items:
             product = cart_item.product
-            line_total = cart_item.line_total
+            line_total_cents = cart_item.line_total_cents
 
             items.append(
                 {
                     "product_id": product.id,
-                    "product_name": product.marketplace_product.title,
+                    "product_name": product.marketplace_product.get_title("en")
+                    if product.marketplace_product
+                    else str(product.id),
                     "quantity": cart_item.quantity,
-                    "price": cart_item.price_at_add,
-                    "line_total": line_total,
+                    "price": cart_item.price_at_add,  # Returns euros via property
+                    "line_total": cents_to_euros(line_total_cents),
                     "image_url": (
                         product.marketplace_product.image_link
                         if product.marketplace_product
@@ -85,8 +91,10 @@ class CartService:
                 }
             )
 
-            subtotal += line_total
+            subtotal_cents += line_total_cents
 
+        # Convert to euros for API response
+        subtotal = cents_to_euros(subtotal_cents)
         cart_data = {
             "vendor_id": vendor_id,
             "session_id": session_id,
@@ -166,8 +174,12 @@ class CartService:
             },
         )
 
-        # Get current price (use sale_price if available, otherwise regular price)
-        current_price = product.sale_price if product.sale_price else product.price
+        # Get current price in cents (use sale_price if available, otherwise regular price)
+        current_price_cents = (
+            product.effective_sale_price_cents
+            or product.effective_price_cents
+            or 0
+        )
 
         # Check if item already exists in cart
         existing_item = (
@@ -236,13 +248,13 @@ class CartService:
                 available=product.available_inventory,
             )
 
-        # Create new cart item
+        # Create new cart item (price stored in cents)
         cart_item = CartItem(
             vendor_id=vendor_id,
             session_id=session_id,
             product_id=product_id,
             quantity=quantity,
-            price_at_add=current_price,
+            price_at_add_cents=current_price_cents,
         )
         db.add(cart_item)
         db.flush()
@@ -253,7 +265,7 @@ class CartService:
             extra={
                 "cart_item_id": cart_item.id,
                 "quantity": quantity,
-                "price": current_price,
+                "price_cents": current_price_cents,
             },
         )