feat(core): add Sequence model

app/models/sequence.py

@@ -0,0 +1,30 @@
+"""Sequence model — document numbering / Nummernkreise."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, Index, Integer, String
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.db import Base, TenantMixin
+
+
+class Sequence(Base, TenantMixin):
+    """Sequence entity for document numbering (e.g. invoice RE-2026-0001)."""
+
+    __tablename__ = "sequences"
+    __table_args__ = (
+        Index("ix_sequences_tenant_name", "tenant_id", "name"),
+    )
+
+    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)
+    prefix: Mapped[str] = mapped_column(String(20), nullable=False, default="")
+    next_number: Mapped[int] = mapped_column(Integer, nullable=False, default=1)
+    padding: Mapped[int] = mapped_column(Integer, nullable=False, default=4)
+    deleted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)