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/httpapi/handlers_config_test.go

@@ -666,6 +666,35 @@ func TestProberRegisterSetsHostnameAndIdleState(t *testing.T) {
 	}
 }
 
+// TestValidatorRegisterSetsHostname proves the admin config listing surfaces
+// a validator's hostname (and heartbeat time) once its agent registers,
+// mirroring TestProberRegisterSetsHostnameAndIdleState for sites.
+func TestValidatorRegisterSetsHostname(t *testing.T) {
+	fc, _, _, _ := newConfigTestHarness(t)
+
+	fc.do(http.MethodPost, "/api/v1/admin/config/validators", createValidatorRequest{ValidatorID: "val-1", OSPortID: "port-1"})
+
+	resp, body := fc.do(http.MethodPost, "/api/v1/agents/register", registerAgentRequest{ValidatorID: "val-1", Hostname: "validator-host-1", AgentVersion: "test"})
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("register agent: status=%d body=%s", resp.StatusCode, body)
+	}
+
+	resp, body = fc.do(http.MethodGet, "/api/v1/admin/config/validators", nil)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("get validators: status=%d body=%s", resp.StatusCode, body)
+	}
+	var validators []validatorDTO
+	if err := json.Unmarshal(body, &validators); err != nil {
+		t.Fatalf("unmarshal validators: %v", err)
+	}
+	if len(validators) != 1 || validators[0].Hostname != "validator-host-1" {
+		t.Fatalf("expected hostname validator-host-1, got %+v", validators)
+	}
+	if validators[0].LastHeartbeatAt != nil {
+		t.Fatalf("expected no heartbeat yet (only register was called), got %+v", validators[0].LastHeartbeatAt)
+	}
+}
+
 // TestProberHeartbeat404UnknownSite proves the heartbeat endpoint rejects
 // an unconfigured site_id, mirroring the validator heartbeat's 404.
 func TestProberHeartbeat404UnknownSite(t *testing.T) {