docs(audit): hash-chain tamper evidence shipped, default off (ScadaBridge#41)

Component-AuditLog Security & Tamper-Evidence describes the per-month chain,
the chaining job and its lock, both purge paths (checkpoint anchor,
tombstones), pre-existing rows, verification and what an unsigned chain does
not prove. alog.md §11.4/§16, CLAUDE.md, HighLevelReqs AL-11/12,
Component-CLI, Component-ManagementService, docs/components/CLI.md and the
CLI README drop the "no-op placeholder" wording.

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

CLAUDE.md

@@ -181,7 +181,8 @@ spec for each is `docs/requirements/Component-<Name>.md`, and `README.md` carrie
 - Payload cap 8 KB by default / 64 KB on error rows; auth headers redacted by default; SQL parameter values captured by default; per-target redaction opt-in. Inbound API: full verbatim capture up to `InboundMaxBytes` (default 1 MiB); request headers stored in `Extra.requestHeaders` (post-redaction); per-method `SkipBodyCapture` flag suppresses bodies while still recording headers + metadata; `AuditInboundCeilingHits` counter surfaced on health snapshot. (M5.3 T7)
 - Audit-write failure NEVER aborts the user-facing action — audit is best-effort, the action's own success/failure path is authoritative.
 - 365-day central retention with monthly partition-switch purge; per-channel retention overrides (`AuditLog:PerChannelRetentionDays`) expire rows earlier than the global window via a bounded, batched row DELETE on the purge actor's maintenance path — values must be shorter than the global window (M5.5 T3); 7-day site SQLite retention with a hard `ForwardState` invariant (no row purged until forwarded or reconciled).
-- Append-only enforced via DB roles (writer role has INSERT only, no UPDATE/DELETE); hash-chain tamper evidence (T1) and Parquet archival (T2) are deferred to v1.x — not shipped in M5.
+- Append-only enforced via DB roles (writer role has INSERT only, no UPDATE/DELETE); Parquet archival (T2) is deferred to v1.x.
+- **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.
 - 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`.