feat(B2): add tenant model + migrations 035/036 + RLS policies

Migration 035: tenants table with 'Schaeffler' default seed.
Migration 036: tenant_id FK on all tables, RLS policies, backfill.
New domains/tenants/ with CRUD router (admin only).
All domain models extended with tenant_id FK.
core/database.py: get_db_for_tenant with RLS context setter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/products/models.py

@@ -1,7 +1,7 @@
 import uuid
 import enum
 from datetime import datetime
-from sqlalchemy import String, DateTime, Boolean, Text, ForeignKey, BigInteger, Enum as SAEnum
+from sqlalchemy import String, DateTime, Boolean, Text, ForeignKey, BigInteger, Enum as SAEnum, Index
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.database import Base
@@ -30,6 +30,9 @@ class CadFile(Base):
     )
     error_message: Mapped[str] = mapped_column(String(2000), nullable=True)
     render_log: Mapped[dict] = mapped_column(JSONB, nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
@@ -61,12 +64,16 @@ class Product(Base):
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     arbeitspaket: Mapped[str | None] = mapped_column(String(500), nullable=True)
     source_excel: Mapped[str | None] = mapped_column(String(1000), nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False
     )
 
     cad_file: Mapped["CadFile | None"] = relationship("CadFile", back_populates="products")
+    tenant: Mapped["Tenant | None"] = relationship("Tenant", back_populates="products", lazy="noload")
     order_lines: Mapped[list["OrderLine"]] = relationship(
         "OrderLine", back_populates="product", cascade="all, delete-orphan"
     )