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>

docs/architecture/money-handling.md

@@ -308,6 +308,97 @@ def test_price_precision():
         assert back_to_euros == price
 ```
 
+## VAT Handling
+
+### Storage Strategy: Gross Prices (VAT-Inclusive)
+
+The platform stores all prices as **gross** (VAT-inclusive) for B2C simplicity:
+
+- Customers see the final price immediately (EU legal requirement)
+- No calculation needed for display
+- Profit is calculated by extracting net from gross
+
+### Luxembourg VAT Rates
+
+| Rate | Percentage | Applies To |
+|------|------------|------------|
+| Standard | **17%** | Most products (electronics, furniture, cosmetics) |
+| Intermediate | **14%** | Wines, printed materials, heating oils |
+| Reduced | **8%** | Utilities, hairdressing, small repairs |
+| Super-reduced | **3%** | Food, books, children's clothing, medicine |
+| Zero | **0%** | Exports, certain financial services |
+
+### Database Fields
+
+```python
+class Product(Base):
+    # Tax rate (0, 3, 8, 14, or 17 for Luxembourg)
+    tax_rate_percent = Column(Integer, default=17, nullable=False)
+
+    # Cost for profit calculation (what vendor pays to acquire)
+    cost_cents = Column(Integer, nullable=True)
+```
+
+### Profit Calculation
+
+```python
+class Product(Base):
+    @property
+    def net_price_cents(self) -> int:
+        """Calculate net price (excluding VAT) from gross price.
+
+        Formula: Net = Gross * 100 / (100 + rate)
+        Example: €119 gross at 17% VAT = €119 * 100 / 117 = €101.71 net
+        """
+        return int(self.effective_price_cents * 100 / (100 + self.tax_rate_percent))
+
+    @property
+    def vat_amount_cents(self) -> int:
+        """VAT = Gross - Net"""
+        return self.effective_price_cents - self.net_price_cents
+
+    @property
+    def profit_cents(self) -> int:
+        """Profit = Net Revenue - Cost"""
+        return self.net_price_cents - self.cost_cents
+
+    @property
+    def profit_margin_percent(self) -> float:
+        """Margin% = (Profit / Net) * 100"""
+        return round((self.profit_cents / self.net_price_cents) * 100, 2)
+```
+
+### Example Calculation
+
+| Field | Value |
+|-------|-------|
+| Gross Price | €119.00 (11900 cents) |
+| Tax Rate | 17% |
+| Net Price | €101.71 (10171 cents) |
+| VAT Amount | €17.29 (1729 cents) |
+| Cost | €60.00 (6000 cents) |
+| **Profit** | €41.71 (4171 cents) |
+| **Margin** | 41.0% |
+
+### Vendor Letzshop Settings
+
+Vendors have default settings for the Letzshop feed:
+
+```python
+class Vendor(Base):
+    # Default VAT rate for new products
+    letzshop_default_tax_rate = Column(Integer, default=17)
+
+    # Product sort priority (0.0-10.0, higher = displayed first)
+    letzshop_boost_sort = Column(String(10), default="5.0")
+
+    # Delivery method: 'nationwide', 'package_delivery', 'self_collect'
+    letzshop_delivery_method = Column(String(100), default="package_delivery")
+
+    # Pre-order days before shipping (default 1 day)
+    letzshop_preorder_days = Column(Integer, default=1)
+```
+
 ## Summary
 
 | Layer | Format | Example |
@@ -318,4 +409,7 @@ def test_price_precision():
 | API request/response | Float euros | `105.91` |
 | Frontend display | Formatted string | `"105,91 €"` |
 
-**Golden Rule:** All arithmetic happens with integers. Conversion to/from euros only at system boundaries (API, display).
+**Golden Rules:**
+1. All arithmetic happens with integers. Conversion to/from euros only at system boundaries.
+2. Prices are stored as gross (VAT-inclusive). Net is calculated when needed.
+3. Tax rate is stored per product, with vendor defaults for new products.