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/materials/models.py

@@ -4,6 +4,10 @@ from sqlalchemy import String, DateTime, Text, ForeignKey, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID
 from app.database import Base
+# TYPE_CHECKING import to avoid circular references
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from app.domains.tenants.models import Tenant
 
 
 class Material(Base):
@@ -17,6 +21,9 @@ class Material(Base):
     created_by: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), 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)
 
@@ -32,6 +39,9 @@ class MaterialAlias(Base):
         UUID(as_uuid=True), ForeignKey("materials.id", ondelete="CASCADE"), nullable=False
     )
     alias: Mapped[str] = mapped_column(String(300), nullable=False)
+    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)
 
     material = relationship("Material", back_populates="aliases")