refactor: no-table deployment authorization

Remove awaiting_approval/rejected status from deployments table and
drop deployment_approvals table. External contributor pushes no longer
create a deployment record — instead handle_push creates a GitHub
Deployment check with failure status linking to an authorize page.

Replace ApproveDeployment/RejectDeployment RPCs with a single
AuthorizeDeployment RPC that takes project_id + branch, fetches HEAD
from GitHub, re-derives deploy contexts, and creates deployments.

Author: Flo4604