feat: add email column to customer_seats table

Add email column to store the seat member's email directly on the seat.
This supports the member_model_enabled feature where seats reference
billing customers rather than individual seat member customers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: psincraian

server/migrations/versions/2026-01-08_add_email_to_customer_seats.py

@@ -0,0 +1,28 @@
+"""add_email_to_customer_seats
+
+Revision ID: 8a7b9c0d1e2f
+Revises: daafaba9088a
+Create Date: 2026-01-08 09:20:32.000000
+
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+# Polar Custom Imports
+
+# revision identifiers, used by Alembic.
+revision = "8a7b9c0d1e2f"
+down_revision = "daafaba9088a"
+branch_labels: tuple[str] | None = None
+depends_on: tuple[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "customer_seats", sa.Column("email", sa.String(length=320), nullable=True)
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("customer_seats", "email")