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/rendering/models.py

@@ -27,6 +27,9 @@ class OutputType(Base):
         Integer, ForeignKey("pricing_tiers.id", ondelete="SET NULL"), nullable=True, index=True
     )
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=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
@@ -53,6 +56,9 @@ class RenderTemplate(Base):
     shadow_catcher_enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
     camera_orbit: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, server_default="true")
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, server_default="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, nullable=False, server_default="now()")
     updated_at: Mapped[datetime] = mapped_column(DateTime, nullable=False, server_default="now()", onupdate=datetime.utcnow)