fix(admin): resolve High code-review findings (Admin-003, Admin-004, Admin-005)

Admin-003 — SignalR hubs were anonymously reachable: an unauthenticated
client could open /hubs/fleet, /hubs/alerts and /hubs/script-log and
stream fleet state, alert detail text and server script-log contents.
Added [Authorize] to FleetStatusHub, AlertHub and ScriptLogHub, and
chained .RequireAuthorization() onto all three MapHub() calls as a
belt-and-braces backstop.

Admin-004 — appsettings.json committed live-looking secrets (the `sa`
ConfigDb password and the LDAP ServiceAccountPassword) in plaintext.
Replaced both with empty placeholders sourced from user-secrets (dev) or
the ConnectionStrings__ConfigDb / Authentication__Ldap__ServiceAccountPassword
environment variables (prod); added a UserSecretsId to the Admin csproj
and a fail-fast guard in Program.cs when ConfigDb is empty/missing.

Admin-005 — Login.razor performed SignInAsync from an interactive Blazor
circuit, where the original HTTP response has long completed so the auth
cookie was not emitted. Rewrote it as a static-rendered plain HTML form
(data-enhance="false") posting to a new AuthEndpoints.MapAuthEndpoints()
minimal-API handler (/auth/login, /auth/logout) that does the LDAP bind,
grant resolution, cookie SignInAsync and redirect while the endpoint
still owns the response. Includes an open-redirect guard on returnUrl.

Added xUnit + Shouldly regression tests: AuthEndpointsTests (login cookie
issuance, failed-bind redirect, open-redirect rejection, logout, anonymous
hub negotiate rejection) and AppSettingsSecretHygieneTests (no committed
secrets). All 26 auth-related tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -1,6 +1,7 @@
 {
+  "_secrets": "Admin-004: no secrets are committed here. Supply the ConfigDb connection string and the LDAP service-account password via user-secrets (dev) or environment variables / a secret store (prod). Env-var keys: ConnectionStrings__ConfigDb and Authentication__Ldap__ServiceAccountPassword. The connection string defaults to Encrypt=True (TLS); use a least-privilege SQL login, not 'sa'.",
   "ConnectionStrings": {
-    "ConfigDb": "Server=10.100.0.35,14330;Database=OtOpcUaConfig;User Id=sa;Password=OtOpcUaDev_2026!;TrustServerCertificate=True;Encrypt=False;"
+    "ConfigDb": ""
   },
   "Authentication": {
     "Ldap": {
@@ -11,7 +12,7 @@
       "AllowInsecureLdap": true,
       "SearchBase": "dc=lmxopcua,dc=local",
       "ServiceAccountDn": "cn=serviceaccount,dc=lmxopcua,dc=local",
-      "ServiceAccountPassword": "serviceaccount123",
+      "ServiceAccountPassword": "",
       "DisplayNameAttribute": "cn",
       "GroupAttribute": "memberOf",
       "GroupToRole": {