feat(core): add TaxRate model

app/models/tax.py

@@ -0,0 +1,31 @@
+"""Tax rate model — ERP tax configuration."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, Index, Numeric, String
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.db import Base, TenantMixin
+
+
+class TaxRate(Base, TenantMixin):
+    """Tax rate entity — e.g. 'Mehrwertsteuer 19%'."""
+
+    __tablename__ = "tax_rates"
+    __table_args__ = (
+        Index("ix_tax_rates_tenant_name", "tenant_id", "name"),
+        Index("ix_tax_rates_tenant_default", "tenant_id", "is_default"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
+    rate: Mapped[float] = mapped_column(Numeric(5, 2), nullable=False)
+    is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    country: Mapped[str | None] = mapped_column(String(2), nullable=True)
+    deleted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)