fix(generic-web): own prod promotion releases

[cbusillo]

Summary

• update generic-web deploy inventory on successful deploys so testing inventory stays current
• reject prod promotion requests that do not match current source inventory
• create or verify GitHub releases from Launchplane after prod promotion passes
• expose release status/tag/url in the prod-promotion API result

Validation

• uv run --extra dev ruff check .
• uv run python -m unittest tests.test_generic_web_promotion tests.test_service.LaunchplaneServiceTests.test_generic_web_prod_promotion_route_executes_for_authorized_product_context tests.test_service.LaunchplaneServiceTests.test_generic_web_prod_promotion_route_accepts_padded_lane_context
• uv run --extra dev mypy control_plane tests
• uv run python -m unittest

Closes #418.
Related: #420.