Retry validator-agent/prober registration; show validator hostname

Both binaries registered with control-api exactly once at startup and
exited (os.Exit(1)) on any failure — including control-api simply not
being up yet (no ordering guarantee between the two at boot/redeploy) or
the admin not having added this validator_id/site_id to the config yet.
Run() now retries registration with capped exponential backoff (3s->30s)
until it succeeds or the process is asked to shut down, instead of
crashing; registerWithRetry is identical in agentcore and probercore
since their Run/register shape already was.

Separately, the admin dashboard's Validators page had no hostname column
even though the agent already reports one on register (mirroring the
prober) and control-api already persists it — only the admin-config read
DTO (validatorDTO in httpapi and dashboard) dropped it before it reached
the template. Added hostname + last_heartbeat_at to that DTO end-to-end
and a Хост/Heartbeat column to validators.html, matching sites.html.

Rebuilt bin/{control-api,admin-dashboard,prober,validator-agent} and
bin/SHA256SUMS per docs/SETUP.md's documented build recipe.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/dashboard/templates/validators.html

@@ -50,12 +50,13 @@
 <div class="panel">
 <div class="table-scroll">
 <table>
-<thead><tr><th>ID</th><th>OS Port ID</th><th>Состояние</th><th></th></tr></thead>
+<thead><tr><th>ID</th><th>Хост</th><th>OS Port ID</th><th>Состояние</th><th>Heartbeat</th><th></th></tr></thead>
 <tbody>
 {{range .Items}}
 {{$b := validatorBadge .State}}
 <tr>
 <td class="mono" data-label="ID">{{.ValidatorID}}</td>
+<td class="mono" data-label="Хост">{{if .Hostname}}{{.Hostname}}{{else}}—{{end}}</td>
 <td data-label="OS Port ID">
 <form class="inline-form" hx-put="/validators/{{.ValidatorID}}" hx-target="#validators-table-wrap" hx-swap="innerHTML">
 <input type="text" name="os_port_id" value="{{.OSPortID}}">
@@ -63,6 +64,7 @@
 </form>
 </td>
 <td data-label="Состояние"><span class="pill {{$b.Class}}">{{$b.Label}}</span></td>
+<td data-label="Heartbeat">{{fmtTime .LastHeartbeatAt}}</td>
 <td data-label="">
 <div class="actions">
 <button class="btn btn-danger-ghost btn-sm" hx-delete="/validators/{{.ValidatorID}}" hx-target="#validators-table-wrap" hx-swap="innerHTML" hx-confirm="Удалить валидатор {{.ValidatorID}}?">Удалить</button>