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/tenants/models.py

@@ -0,0 +1,21 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import String, DateTime, Boolean
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.database import Base
+
+
+class Tenant(Base):
+    __tablename__ = "tenants"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name: Mapped[str] = mapped_column(String(200), nullable=False)
+    slug: Mapped[str] = mapped_column(String(100), nullable=False, unique=True)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+
+    # Relationships (lazy=noload — loaded explicitly when needed)
+    users: Mapped[list] = relationship("User", back_populates="tenant", lazy="noload")
+    orders: Mapped[list] = relationship("Order", back_populates="tenant", lazy="noload")
+    products: Mapped[list] = relationship("Product", back_populates="tenant", lazy="noload")