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/schemas.py

@@ -0,0 +1,26 @@
+import uuid
+from datetime import datetime
+from pydantic import BaseModel
+
+
+class TenantCreate(BaseModel):
+    name: str
+    slug: str
+    is_active: bool = True
+
+
+class TenantUpdate(BaseModel):
+    name: str | None = None
+    slug: str | None = None
+    is_active: bool | None = None
+
+
+class TenantOut(BaseModel):
+    id: uuid.UUID
+    name: str
+    slug: str
+    is_active: bool
+    user_count: int | None = None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}