agent+server: apply host bandwidth caps to restic invocations

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.

internal/server/http/host_credentials.go

@@ -399,6 +399,10 @@ func (s *Server) pushAdminCredsToAgent(ctx context.Context, hostID string) error
 // don't race a brand-new register against an old still-closing conn.
 func (s *Server) onAgentHello(ctx context.Context, hostID string, conn *ws.Conn) {
 	s.pushRepoCredsOnHello(ctx, hostID, conn)
+	// Bandwidth caps are sent unconditionally so an agent that
+	// reconnects after a cap edit picks up the new state without
+	// waiting for the next bandwidth PUT.
+	s.pushBandwidthOnHello(ctx, hostID, conn)
 	// Push the current schedule set in the same on-hello window so
 	// the agent's local cron is in sync before any command.run lands.
 	// An empty schedule list is a valid push: it tells the agent to