feat(T06): Verkaufsmodul + Rechtsdokumente + DATEV-Export + Sales UI

backend/app/utils/datev.py

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+"""DATEV CSV format helper (Buchungsstapel).
+
+Generates CSV in DATEV-compatible format with headers:
+Datum, Konto, Gegenkonto, Betrag, Belegfeld, Buchungstext
+"""
+
+import csv
+import io
+from datetime import date
+from decimal import Decimal
+from typing import Any
+
+
+# DATEV Buchungsstapel CSV headers
+DATEV_HEADERS = [
+    "Datum",
+    "Konto",
+    "Gegenkonto",
+    "Betrag",
+    "Belegfeld",
+    "Buchungstext",
+]
+
+
+def generate_datev_csv(sales: list[Any]) -> str:
+    """Generate DATEV CSV content from a list of sale objects.
+
+    Each sale produces one booking row:
+    - Datum: sale_date in DD.MM.YYYY format
+    - Konto: buyer account (1200 = Forderungen aus Lieferungen/Leistungen)
+    - Gegenkonto: revenue account (8400 = Erlöse aus Warenverkäufen)
+    - Betrag: sale_price as decimal with comma separator
+    - Belegfeld: sale ID (short form)
+    - Buchungstext: vehicle make + model + FIN
+
+    Args:
+        sales: List of Sale model instances with nested vehicle and buyer.
+
+    Returns:
+        CSV string with DATEV headers and one row per sale.
+    """
+    output = io.StringIO()
+    writer = csv.writer(output, delimiter=";", quoting=csv.QUOTE_MINIMAL)
+
+    # Write header row
+    writer.writerow(DATEV_HEADERS)
+
+    for sale in sales:
+        # Format date as DD.MM.YYYY (DATEV convention)
+        datum = sale.sale_date.strftime("%d.%m.%Y") if sale.sale_date else ""
+
+        # Account numbers (standard DATEV SKR03)
+        konto = "1200"  # Forderungen aus LuL
+        gegenkonto = "8400"  # Erlöse aus Warenverkäufen
+
+        # Amount with comma as decimal separator (DATEV convention)
+        betrag = _format_amount(sale.sale_price)
+
+        # Belegfeld: short sale ID (first 8 chars)
+        belegfeld = str(sale.id)[:8].upper()
+
+        # Buchungstext: vehicle description
+        vehicle = getattr(sale, "vehicle", None)
+        if vehicle:
+            buchungstext = f"{vehicle.make} {vehicle.model} {vehicle.fin}"
+        else:
+            buchungstext = f"Verkauf {str(sale.id)[:8]}"
+
+        writer.writerow([datum, konto, gegenkonto, betrag, belegfeld, buchungstext])
+
+    return output.getvalue()
+
+
+def _format_amount(amount: Decimal) -> str:
+    """Format a Decimal amount for DATEV CSV (comma as decimal separator)."""
+    if amount is None:
+        return "0,00"
+    # Convert to string with 2 decimal places, replace dot with comma
+    formatted = f"{amount:.2f}"
+    return formatted.replace(".", ",")
+
+
+def validate_datev_csv(csv_content: str) -> bool:
+    """Validate that CSV content has correct DATEV headers and structure.
+
+    Args:
+        csv_content: CSV string to validate.
+
+    Returns:
+        True if valid, False otherwise.
+    """
+    if not csv_content or not csv_content.strip():
+        return False
+
+    reader = csv.reader(io.StringIO(csv_content), delimiter=";")
+    try:
+        header = next(reader)
+    except StopIteration:
+        return False
+
+    if header != DATEV_HEADERS:
+        return False
+
+    # Check at least that rows have 6 columns each
+    for row in reader:
+        if len(row) != 6:
+            return False
+
+    return True