phase 1: agent install path — systemd unit, install.sh, asset endpoints

P1-14 deploy/install/restic-manager-agent.service: standard systemd
  unit with the usual hardening switches (NoNewPrivileges, Protect*,
  RestrictRealtime, MemoryDenyWriteExecute). Restart=always with a
  5s backoff. Runs as a dedicated unprivileged restic-manager-agent
  user; the install script creates it.

P1-29 deploy/install/install.sh: arch detection (amd64/arm64), pulls
  the agent binary from /agent/binary, creates the service user
  + dirs (/etc/restic-manager, /var/lib/restic-manager), runs
  enrollment via `agent -enroll-server -enroll-token`, lays down
  the systemd unit, enables and starts it.

  Honours the spec's "detect, don't auto-disable" rule for existing
  schedulers: scans systemd timers, /etc/cron.d/*, /etc/cron.daily/*,
  root crontab for restic-named entries and prints them with the
  exact disable command — operator decides.

P1-31 server endpoints to ship the agent installation payload:
  GET /agent/binary?os=linux&arch=amd64 → serves
    <DataDir>/agent-binaries/restic-manager-agent-linux-amd64
  GET /install/<file>                   → serves
    <DataDir>/install/<file>
  Both endpoints reject path traversal and return 404 if the file
  isn't published. Operators drop the binaries + service unit into
  these directories at release time. Signed-bundle verification is
  deferred to Phase 5 OSS readiness.

All tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/http/server.go

@@ -93,6 +93,12 @@ func (s *Server) routes(r chi.Router) {
 		}))
 	}
 
+	// Agent binaries + install scripts. Open endpoints — content is
+	// unprivileged on its own, gating happens via the enrollment
+	// token. See agent_assets.go.
+	r.Get("/agent/binary", s.handleAgentBinary)
+	r.Get("/install/*", s.handleInstallAsset)
+
 	// UI handlers will hang off / — Phase 1 will add them.
 	r.Get("/", func(w stdhttp.ResponseWriter, _ *stdhttp.Request) {
 		_, _ = fmt.Fprint(w, "restic-manager — UI not yet implemented")