feat: add VAT tax rate, cost, and Letzshop feed settings

Product Model:
- Add tax_rate_percent (NOT NULL, default 17) for Luxembourg VAT
- Add cost_cents for profit calculation
- Add profit calculation properties: net_price, vat_amount, profit, margin
- Rename supplier_cost_cents to cost_cents

MarketplaceProduct Model:
- Add tax_rate_percent (NOT NULL, default 17)

Vendor Model (Letzshop feed settings):
- letzshop_default_tax_rate: Default VAT for new products (0, 3, 8, 14, 17)
- letzshop_boost_sort: Product sort priority (0.0-10.0)
- letzshop_delivery_method: nationwide, package_delivery, self_collect
- letzshop_preorder_days: Pre-order shipping delay

VAT Strategy:
- Store prices as gross (VAT-inclusive) for B2C
- Calculate net from gross when needed for profit
- Luxembourg VAT rates: 0%, 3%, 8%, 14%, 17%

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

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

models/database/marketplace_product.py

@@ -100,6 +100,11 @@ class MarketplaceProduct(Base, TimestampMixin):
     sale_price_cents = Column(Integer)  # Parsed numeric sale price in cents
     currency = Column(String(3), default="EUR")
 
+    # === TAX / VAT ===
+    # Luxembourg VAT rates: 0 (exempt), 3 (super-reduced), 8 (reduced), 14 (intermediate), 17 (standard)
+    # Prices are stored as gross (VAT-inclusive). Default to standard rate.
+    tax_rate_percent = Column(Integer, default=17, nullable=False)
+
     # === MEDIA ===
     image_link = Column(String)
     additional_image_link = Column(String)  # Legacy single string