docs(secrets): G-4 ${secret:} config delivery convention + fail-closed proof (Task 4)

OtOpcUa commits no plaintext secrets — the only secret-shaped committed value is
ServerHistorian:ApiKey='' (empty, disabled by default). Because the pre-host expander
is fail-closed AND section-agnostic, hard-committing a ${secret:} token would break
every dev/CI/integration boot for zero at-rest benefit. Instead document the token
delivery convention for all five config secrets (jwt/ldap/deploy/configdb/historian)
in docs/security.md and note the token option in the ServerHistorian comments.

Mechanism proven live: unseeded token boot fails with SecretNotFoundException at
pre-host expansion; seeded token resolves and boot proceeds. Behavior-neutral
(comment + doc only).

src/Server/ZB.MOM.WW.OtOpcUa.Host/appsettings.json

@@ -25,7 +25,7 @@
     "Enabled": false,
     "Endpoint": "",
     "ApiKey": "",
-    "_ApiKeyComment": "NEVER commit a real key. Supply via the environment variable ServerHistorian__ApiKey.",
+    "_ApiKeyComment": "NEVER commit a real key. Supply via env var ServerHistorian__ApiKey, either as a literal or as a ${secret:otopcua/historian/api-key} token resolved fail-closed by the pre-host secrets expander.",
     "UseTls": true,
     "AllowUntrustedServerCertificate": false,
     "CaCertificatePath": null,