Type product context cutover store boundary

[cbusillo]

Summary

• add structural read/apply store protocols for product context cutover and legacy cleanup
• remove the concrete Postgres store dependency from cutover planning/apply helpers
• make cutover apply and cleanup loops use record-specific variable names so mypy can verify each record shape
• add a fake-store dry-run planning test proving the read-only cutover boundary without a database

Refs #161

Validation

• uv run python -m unittest tests.test_product_context_cutover
• uv run --extra dev mypy control_plane/product_context_cutover.py tests/test_product_context_cutover.py
• uv run --extra dev ruff check --diff control_plane/product_context_cutover.py tests/test_product_context_cutover.py
• uv run --extra dev ruff format --check --diff control_plane/product_context_cutover.py tests/test_product_context_cutover.py
• git diff --check
• uv run python -m unittest