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/db.go

@@ -46,6 +46,9 @@ var verdictIntegritySchema string
 //go:embed migrations/0011_check_runs.sql
 var checkRunsSchema string
 
+//go:embed migrations/0012_self_check_failures.sql
+var selfCheckFailuresSchema string
+
 // migrations is the ordered list of schema versions. Each entry's SQL is
 // applied, in order, for any version greater than the database's current
 // PRAGMA user_version — so a fresh database walks the whole list and an
@@ -65,6 +68,7 @@ var migrations = []struct {
 	{9, scaleIndexesSchema},
 	{10, verdictIntegritySchema},
 	{11, checkRunsSchema},
+	{12, selfCheckFailuresSchema},
 }
 
 type DB struct {