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/orders/models.py

@@ -33,9 +33,13 @@ class Order(Base):
     completed_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
     rejected_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True)
     estimated_price: Mapped[Decimal | None] = mapped_column(Numeric(12, 2), nullable=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True
+    )
 
     template: Mapped["Template"] = relationship("Template", back_populates="orders")
     created_by_user: Mapped["User"] = relationship("User", back_populates="orders", foreign_keys=[created_by])
+    tenant: Mapped["Tenant | None"] = relationship("Tenant", back_populates="orders", lazy="noload")
     items: Mapped[list["OrderItem"]] = relationship("OrderItem", back_populates="order", cascade="all, delete-orphan")
     lines: Mapped[list["OrderLine"]] = relationship(
         "OrderLine", back_populates="order", cascade="all, delete-orphan"
@@ -92,6 +96,9 @@ class OrderItem(Base):
 
     item_status: Mapped[ItemStatus] = mapped_column(SAEnum(ItemStatus), default=ItemStatus.pending, nullable=False)
     notes: Mapped[str] = mapped_column(Text, 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)
 
@@ -137,6 +144,9 @@ class OrderLine(Base):
         nullable=True,
     )
     notes: Mapped[str | None] = mapped_column(Text, 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