Adds POST /webhooks/brain-sync: verifies Gitea's HMAC-SHA256 signature,
checks the push is to mathias/brain main, then creates a one-off Job from
the existing brain-sync CronJob's template (same script the 15-min poll
already runs, just triggered on-demand). Off by default -- opt in via
GITEA_WEBHOOK_SECRET, since it needs Job-create RBAC in the "brain"
namespace a fresh deploy won't have.
10 new tests (internal/webhook), including a fake-clientset reactor to
simulate server-side GenerateName expansion, which the plain fake tracker
doesn't do on its own.
Needs (follow-up, infra repo): RBAC granting ingestion's ServiceAccount
get on cronjobs/brain-sync + create on jobs in the brain namespace, the
GITEA_WEBHOOK_SECRET env, and the actual Gitea webhook registration.
