Retry a failed self-check on another validator; add the self-check failure ceiling

A validator that failed the self-check of an address no longer gets that address
again in the current round (ClaimNextQueued skips it); the validator itself stays
in service and takes all other addresses. The verdict fail is set when the number
of failed self-checks of an address reaches settings.self_check_max_attempts
(1..50, default 5, independent of the number of validators); max_retries and
retry_count are no longer used for self-check. If every working validator has
already failed the address, a new round starts and the exclusions lapse.

Migration 0012: ip_self_check_failures (permanent history per registry address),
ip_queue.sc_failures and sc_round_start_cycle (cycle_id is used instead of
attempt_number, which restarts when a queue row is recreated), the setting.
db.FailSelfCheck does it in one transaction; re-submission starts a new series.
API: self_check_max_attempts in GET/PUT /admin/config/orchestrator,
self_check_failed_on in /admin/ips/{ip} and /admin/registry/{ip}. Dashboard: the
field on /settings and the line "Self-check не прошёл на: ..." on the address
pages. Docs, plan and summary in docs/changes/.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/db/models.go

@@ -276,10 +276,20 @@ type Settings struct {
 	// HistoryRetentionCycles caps how many recent check cycles are kept per
 	// registry address (see PruneRegistryHistory); 0 means unlimited.
 	HistoryRetentionCycles int
-	CreatedAt              time.Time
-	UpdatedAt              time.Time
+	// SelfCheckMaxAttempts is the ceiling of failed self-checks per address
+	// (1..MaxSelfCheckMaxAttempts); reaching it gives the address the verdict
+	// fail (see FailSelfCheck).
+	SelfCheckMaxAttempts int
+	CreatedAt            time.Time
+	UpdatedAt            time.Time
 }
 
+// Bounds of Settings.SelfCheckMaxAttempts.
+const (
+	MinSelfCheckMaxAttempts = 1
+	MaxSelfCheckMaxAttempts = 50
+)
+
 // InboundChecksSettings is the singleton row describing what the prober
 // checks on every site for every in-flight IP (TCP ports + optional ICMP).
 // Admin-configurable at runtime (see queries_inbound.go).