P2R-13a. restic.Env gains LimitUploadKBps/LimitDownloadKBps which are
emitted as global --limit-upload/--limit-download flags before the
subcommand on every invocation. Agent dispatcher tracks host-wide
caps received via config.update; server pushes them on hello and
after PUT /api/hosts/{id}/bandwidth.
Also extends api.CommandRunPayload with optional per-job overrides
(BandwidthUpKBps/Down + PreHook/PostHook); the override consumers
land in T2/T6.
