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/alembic/versions/035_tenants.py

@@ -0,0 +1,36 @@
+"""Add tenants table.
+
+Revision ID: 035
+Revises: 034
+Create Date: 2026-03-06
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import UUID
+
+revision = '035'
+down_revision = '034'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        'tenants',
+        sa.Column('id', UUID(as_uuid=True), primary_key=True,
+                  server_default=sa.text('gen_random_uuid()')),
+        sa.Column('name', sa.String(200), nullable=False),
+        sa.Column('slug', sa.String(100), nullable=False, unique=True),
+        sa.Column('is_active', sa.Boolean(), nullable=False, server_default='true'),
+        sa.Column('created_at', sa.DateTime(), nullable=False,
+                  server_default=sa.text('NOW()')),
+    )
+    # Seed default tenant — all existing data will be assigned to this tenant
+    op.execute("""
+        INSERT INTO tenants (name, slug, is_active)
+        VALUES ('Schaeffler', 'schaeffler', true)
+    """)
+
+
+def downgrade():
+    op.drop_table('tenants')