p6-01/02: agent self-update + fleet update server cluster

- alert: update_failed (per-host, dedup=hostID) + fleet_update_halted
  (system-scoped, host_id NULL via new RaiseOrTouchSystem helper).
- ws: UpdateWatcher tracks in-flight command.update dispatches and
  reconciles them against incoming hello envelopes — success path
  marks the job succeeded and auto-resolves the alert; 90s timeout
  marks the job failed and raises update_failed.
- http: POST /api/hosts/{id}/update (admin-only JSON) + the HTMX
  /hosts/{id}/update form variant. Pre-checks: host exists, online,
  agent_version != current, no running update job. Refactored core
  into Server.dispatchHostUpdate so the fleet worker can share it
  without going through HTTP.
- fleetupdate: rolling worker iterating through host slots, halting
  on first failure and raising fleet_update_halted. Polling-based
  version-match (re-read hosts.agent_version every 1s up to 95s) —
  no extra plumbing into the WS hello path. At-most-one-running is
  enforced at the store layer (ErrFleetUpdateRunning).
- cmd/server: wire UpdateWatcher and FleetWorker into the main
  goroutine; the worker uses a small serverDispatcher adapter that
  delegates back into Server.DispatchHostUpdate.

Tests: watcher (success/timeout/mismatch/late-hello), HTTP endpoint
(happy + four pre-check branches + RBAC), worker (two-host happy,
timeout-halt, host-offline-halt, already-at-target skip, cancel
mid-run, double-Start guard).

internal/server/ws/handler.go

@@ -16,6 +16,7 @@ import (
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/auth"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
 )
 
 // HandlerDeps is the set of collaborators the agent WS handler needs.
@@ -26,6 +27,9 @@ type HandlerDeps struct {
 	// AlertEngine receives job-finished and host-online events so the
 	// alert engine can evaluate its rules. Optional; nil = no-op.
 	AlertEngine *alert.Engine
+	// UpdateWatcher reconciles in-flight agent-update dispatches against
+	// hello envelopes. Optional; nil = no-op.
+	UpdateWatcher *UpdateWatcher
 	// OnHello is called once per successful hello, after the host row
 	// has been touched and the conn registered. Used by the HTTP
 	// layer to push host_credentials down as a config.update before
@@ -147,6 +151,9 @@ func runAgentLoop(ctx context.Context, c *Conn, hostID string, deps HandlerDeps)
 	if deps.AlertEngine != nil {
 		deps.AlertEngine.NotifyHostOnline(hostID)
 	}
+	if deps.UpdateWatcher != nil {
+		deps.UpdateWatcher.OnHello(ctx, hostID, helloPayload.AgentVersion, version.Version)
+	}
 
 	deps.Hub.Register(hostID, c)
 	defer deps.Hub.Unregister(hostID, c)