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/billing/models.py

@@ -1,8 +1,13 @@
+import uuid
 from datetime import datetime
 from decimal import Decimal
-from sqlalchemy import String, Boolean, DateTime, Text, Numeric, Integer, UniqueConstraint, Index
+from sqlalchemy import String, Boolean, DateTime, Text, Numeric, Integer, UniqueConstraint, Index, ForeignKey
 from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
 from app.database import Base
+from typing import TYPE_CHECKING
+if TYPE_CHECKING:
+    from app.domains.tenants.models import Tenant
 
 
 class PricingTier(Base):
@@ -14,6 +19,9 @@ class PricingTier(Base):
     price_per_item: Mapped[Decimal] = mapped_column(Numeric(10, 2), nullable=False)
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
     is_active: Mapped[bool] = mapped_column(Boolean, default=True, 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)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)