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>

models/database/cart.py

@@ -1,9 +1,12 @@
 # models/database/cart.py
-"""Cart item database model."""
+"""Cart item database model.
+
+Money values are stored as integer cents (e.g., €105.91 = 10591).
+See docs/architecture/money-handling.md for details.
+"""
 
 from sqlalchemy import (
     Column,
-    Float,
     ForeignKey,
     Index,
     Integer,
@@ -13,6 +16,7 @@ from sqlalchemy import (
 from sqlalchemy.orm import relationship
 
 from app.core.database import Base
+from app.utils.money import cents_to_euros, euros_to_cents
 from models.database.base import TimestampMixin
 
 
@@ -22,6 +26,8 @@ class CartItem(Base, TimestampMixin):
 
     Stores cart items per session, vendor, and product.
     Sessions are identified by a session_id string (from browser cookies).
+
+    Price is stored as integer cents for precision.
     """
 
     __tablename__ = "cart_items"
@@ -33,7 +39,7 @@ class CartItem(Base, TimestampMixin):
 
     # Cart details
     quantity = Column(Integer, nullable=False, default=1)
-    price_at_add = Column(Float, nullable=False)  # Store price when added to cart
+    price_at_add_cents = Column(Integer, nullable=False)  # Price in cents when added
 
     # Relationships
     vendor = relationship("Vendor")
@@ -49,7 +55,24 @@ class CartItem(Base, TimestampMixin):
     def __repr__(self):
         return f"<CartItem(id={self.id}, session='{self.session_id}', product_id={self.product_id}, qty={self.quantity})>"
 
+    # === PRICE PROPERTIES (Euro convenience accessors) ===
+
+    @property
+    def price_at_add(self) -> float:
+        """Get price at add in euros."""
+        return cents_to_euros(self.price_at_add_cents)
+
+    @price_at_add.setter
+    def price_at_add(self, value: float):
+        """Set price at add from euros."""
+        self.price_at_add_cents = euros_to_cents(value)
+
+    @property
+    def line_total_cents(self) -> int:
+        """Calculate line total in cents."""
+        return self.price_at_add_cents * self.quantity
+
     @property
     def line_total(self) -> float:
-        """Calculate line total."""
-        return self.price_at_add * self.quantity
+        """Calculate line total in euros."""
+        return cents_to_euros(self.line_total_cents)