feat(11-01): create sites table migration, model, and schemas

- Add migration 030 with sites table, RLS policy, and device site_id FK
- Add Site SQLAlchemy model with tenant isolation
- Add site_id nullable FK and relationship to Device model
- Add sites relationship to Tenant model
- Register Site in models __init__.py
- Add SiteCreate, SiteUpdate, SiteResponse, SiteListResponse schemas

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/030_create_sites_table.py

@@ -0,0 +1,94 @@
+"""Create sites table with RLS and add site_id FK to devices.
+
+Revision ID: 030
+Revises: 029
+Create Date: 2026-03-19
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "030"
+down_revision = "029"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # 1. Create sites table
+    op.create_table(
+        "sites",
+        sa.Column(
+            "id",
+            sa.dialects.postgresql.UUID(as_uuid=True),
+            primary_key=True,
+            server_default=sa.text("gen_random_uuid()"),
+        ),
+        sa.Column(
+            "tenant_id",
+            sa.dialects.postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("tenants.id", ondelete="CASCADE"),
+            nullable=False,
+            index=True,
+        ),
+        sa.Column("name", sa.String(255), nullable=False),
+        sa.Column("latitude", sa.Float, nullable=True),
+        sa.Column("longitude", sa.Float, nullable=True),
+        sa.Column("address", sa.Text, nullable=True),
+        sa.Column("elevation", sa.Float, nullable=True),
+        sa.Column("notes", sa.Text, nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+        sa.UniqueConstraint("tenant_id", "name", name="uq_sites_tenant_name"),
+    )
+
+    # 2. Enable RLS on sites table
+    conn = op.get_bind()
+    conn.execute(sa.text("ALTER TABLE sites ENABLE ROW LEVEL SECURITY"))
+    conn.execute(sa.text("ALTER TABLE sites FORCE ROW LEVEL SECURITY"))
+    conn.execute(
+        sa.text("""
+            CREATE POLICY tenant_isolation ON sites
+            USING (
+                tenant_id::text = current_setting('app.current_tenant', true)
+                OR current_setting('app.current_tenant', true) = 'super_admin'
+            )
+            WITH CHECK (
+                tenant_id::text = current_setting('app.current_tenant', true)
+                OR current_setting('app.current_tenant', true) = 'super_admin'
+            )
+        """)
+    )
+
+    # 3. Add nullable site_id FK column to devices table
+    op.add_column(
+        "devices",
+        sa.Column(
+            "site_id",
+            sa.dialects.postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("sites.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )
+    op.create_index("ix_devices_site_id", "devices", ["site_id"])
+
+
+def downgrade() -> None:
+    # Drop devices.site_id column (index drops automatically with column)
+    op.drop_index("ix_devices_site_id", table_name="devices")
+    op.drop_column("devices", "site_id")
+
+    # Drop RLS policy and sites table
+    conn = op.get_bind()
+    conn.execute(sa.text("DROP POLICY IF EXISTS tenant_isolation ON sites"))
+    op.drop_table("sites")