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

src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -531,7 +531,12 @@ try
 
         var configDbConnectionString = configuration["ScadaBridge:Database:ConfigurationDb"]
             ?? throw new InvalidOperationException("ScadaBridge:Database:ConfigurationDb connection string is required for Central role.");
-        builder.Services.AddConfigurationDatabase(configDbConnectionString);
+        // Optional second login for the AuditLog maintenance paths (chain job + both purge
+        // paths, ScadaBridge#107 item 3). Read from the pre-host-expanded configuration, so it
+        // may be a ${secret:} reference; unset = everything on the runtime connection, as before.
+        builder.Services.AddConfigurationDatabase(
+            configDbConnectionString,
+            configuration[ZB.MOM.WW.ScadaBridge.AuditLog.ServiceCollectionExtensions.MaintenanceConnectionStringKey]);
         builder.Services.AddOptions<ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Maintenance.ConfigAuditOptions>()
             .Bind(configuration.GetSection(ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Maintenance.ConfigAuditOptions.SectionName))
             .ValidateOnStart();