Expose generic web rollback planner route

[shiny-code-bot]

Summary

• expose POST /v1/drivers/generic-web/prod-rollback-plan as a safe-write generic driver action
• wire the route through descriptor authz, base-driver inheritance, idempotency, and the existing rollback plan contract
• document the rollback planner route and add service/descriptor coverage

Validation

• uv run python -m unittest tests.test_driver_descriptors tests.test_generic_web_rollback tests.test_service
• uv run python -m unittest tests.test_driver_descriptors.DriverDescriptorRegistryTests.test_generic_web_descriptor_is_provider_neutral_base_driver tests.test_driver_descriptors.DriverDescriptorRegistryTests.test_generic_web_execution_metadata_matches_descriptors tests.test_service.LaunchplaneServiceTests.test_generic_web_rollback_plan_route_writes_plan_record tests.test_service.LaunchplaneServiceTests.test_generic_web_rollback_plan_route_rejects_unauthorized_context
• uv run --extra dev ruff check --diff control_plane/service.py control_plane/drivers/registry.py tests/test_driver_descriptors.py tests/test_service.py
• uv run --extra dev ruff check control_plane/service.py control_plane/drivers/registry.py tests/test_driver_descriptors.py tests/test_service.py
• uv run --extra dev ruff format --check control_plane/service.py control_plane/drivers/registry.py tests/test_driver_descriptors.py tests/test_service.py
• uv run --extra dev mypy control_plane tests

Notes

• JetBrains inspection returned only STATUS: error; no actionable findings were available from the tool.