feat(audit): off-box chain-head export, verify-heads, maintenance login (ScadaBridge#107)

Item 1 (owner decision: no signing). AuditLog:HashChain:HeadExport (default
off): both centrals periodically read every month's chain head (LastSeq,
LastHash, purge anchor, chained rows, tombstones; one snapshot under the
chain lock SHARED) and append the changed heads to a sink the DB login cannot
write - File (append-only JSONL per node per day, local or UNC) or Syslog
(RFC 5424 over UDP / TCP octet counting). `audit verify-heads --path` reads
those records back (file or syslog-collector lines) and the new
VerifyAuditChainHeads command checks each against the database: Matches /
Extended / Purged, or Mismatch / Regressed / MissingHead / MissingLink /
CountMismatch (exit 3). AuditChainHeadStoreTests forges a chain with the
production hasher: verify-chain says Intact, verify-heads says Mismatch.

Item 3. AuditLog:MaintenanceConnectionString (${secret:}-capable, unset =
today's single login) runs the chain job, chain verify / head export, the
partition switch-out + archive and the per-channel purge on
AuditMaintenanceContext (keyed audit-maintenance repository/archive, purge
actor prefers them). AuditMaintenanceLoginTests runs the hardened setup on
SQL Server: writer (datareader/datawriter/audit_writer) ingests and is
refused chaining and deleting; the chainer+purger login does the full cycle.
Two-login grants documented in Component-AuditLog.md.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

CLAUDE.md

@@ -184,7 +184,7 @@ spec for each is `docs/requirements/Component-<Name>.md`, and `README.md` carrie
 - Append-only enforced via DB roles (writer role has INSERT only, no UPDATE/DELETE).
 - **Clustered key `(OccurredAtUtc, EventId)` + aligned unique `UX_AuditLog_EventId_Occurred (EventId, OccurredAtUtc)` (migration `FlipAuditLogClusteredKey`, #111, B2 of the #78 benchmark).** The Audit Log page's newest-first page/keyset now reads an ordered range instead of the whole table (300k rows: 90,532 → 34 reads). Offline whole-table rebuild — script `docs/plans/sql/FlipAuditLogClusteredKey.sql` (+ `.rollback.sql`), both centrals stopped: the partition-switch staging table now reads the live key order, but binaries older than the flip hard-code the old one. DEV/QA not applied (sizing + window in #111).
 - **Parquet cold archive (T2, ScadaBridge#42) — default OFF (`AuditLog:Archive:Enabled` + absolute `Directory`, shared by both centrals).** The purge actor writes each expired partition to `{Directory}/{YYYY}/auditlog-{YYYY-MM}-{stamp}Z.parquet` (10 stored columns + `ChainSeq`/`RowHash`, tick-exact `OccurredAtUtcTicks`) plus `.tombstones.parquet` and a `.manifest.json` (written last; chain head at archive time, SHA-256 per file), read under the chain lock SHARED; then the switch runs with a guard (under the EXCLUSIVE lock: partition row count + each month's `LastSeq` must still equal the archive's, else THROW 51002/51003 and nothing is purged). Fail closed: a partition that cannot be archived is not purged. `audit archive list|verify` = `ListAuditArchives`/`VerifyAuditArchive` (verify-chain's gate): file hashes, chain recomputed from the archive, chain end vs the DB purge anchor (`Matches`/`Superseded`/`NotPurged`/`Mismatch`), CLI exit 3. `audit export --format parquet` streams the 24 export columns (was 501). No migration; no re-import path (open decision), object storage not wired (mount it).
-- **Hash-chain tamper evidence (T1, ScadaBridge#41) — default OFF (`AuditLog:HashChain:Enabled`).** One SHA-256 chain per UTC calendar month: `ChainSeq` + `RowHash = SHA-256(prev || canonical(row))` on `AuditLog` (migration `AddAuditLogHashChain`), linked by the central `AuditLogHashChainService` (both nodes, serialised by `sp_getapplock`) — never by the ingest path. `AuditLogChainHead` catches tail deletion and carries the partition-switch checkpoint anchor; the per-channel DELETE writes an `AuditLogChainTombstone` per chained row in the same transaction. `audit verify-chain --month` = management command `VerifyAuditChain` (OperationalAudit, unscoped callers only), first break with partition/ChainSeq/EventId, CLI exit 3. A chained row's `SourceNode` is frozen (backfill skips it). Unsigned; signing/external anchoring are open.
+- **Hash-chain tamper evidence (T1, ScadaBridge#41) — default OFF (`AuditLog:HashChain:Enabled`).** One SHA-256 chain per UTC calendar month: `ChainSeq` + `RowHash = SHA-256(prev || canonical(row))` on `AuditLog` (migration `AddAuditLogHashChain`), linked by the central `AuditLogHashChainService` (both nodes, serialised by `sp_getapplock`) — never by the ingest path. `AuditLogChainHead` catches tail deletion and carries the partition-switch checkpoint anchor; the per-channel DELETE writes an `AuditLogChainTombstone` per chained row in the same transaction. `audit verify-chain --month` = management command `VerifyAuditChain` (OperationalAudit, unscoped callers only), first break with partition/ChainSeq/EventId, CLI exit 3. A chained row's `SourceNode` is frozen (backfill skips it). **Unsigned by owner decision (#107); external anchor = off-box head export** (`AuditLog:HashChain:HeadExport`, default off: both centrals append changed per-month heads — month, `LastSeq`/`LastHash`, purge anchor, row + tombstone counts — to append-only JSONL files `{Directory}/{yyyy}/audit-chain-heads-{date}-{node}.jsonl` or RFC 5424 syslog UDP/TCP); `audit verify-heads --path` = `VerifyAuditChainHeads` checks them against the DB (a re-hashed forgery passes verify-chain, fails verify-heads; exit 3). **Two-login hardening (#107 item 3):** `AuditLog:MaintenanceConnectionString` (`${secret:}`-capable, unset = runtime login) runs the chain job, verify/head export, partition switch-out + archive and per-channel purge on `AuditMaintenanceContext` (keyed `audit-maintenance` repository/archive); that login gets `scadabridge_audit_chainer` + `scadabridge_audit_purger`, the runtime login `scadabridge_audit_writer` (whose DENYs block chaining/purging) — pinned by `AuditMaintenanceLoginTests`.
 - Node-of-origin is captured alongside site-of-origin: `SourceNode` (`varchar(64)` NULL) on `AuditLog`, `Notifications`, and `SiteCalls` — `node-a`/`node-b` for site rows (qualified by `SourceSiteId`/`SourceSite`), `central-a`/`central-b` for central direct-write rows. Stamped at the writing node, carried verbatim through telemetry + reconciliation, and indexed via `IX_AuditLog_Node_Occurred (SourceNode, OccurredAtUtc)` on `AuditLog`.
 - Per-node stuck KPIs (M5.3 T6): Notification Outbox and Site Call Audit expose `PerNodeNotificationKpiRequest`/`PerNodeSiteCallKpiRequest` messages that group stuck/parked/delivered counts by `SourceNode`, surfacing per-node breakdowns on the Health dashboard.
 - `audit tree --execution-id <guid>` CLI command (M5.3 T8) + `GET /api/audit/tree` endpoint — resolves any node to its chain root and renders the full execution tree; backed by `IAuditLogRepository.GetExecutionTreeAsync`.