fail startup when the quick connect store is unreachable

Read the configured quick connect store during InitializeServices and abort
startup, with a log line naming valkey, instead of coming up and failing on
the first request that needs the store.

Emby.Server.Implementations/ApplicationHost.cs

@@ -124,6 +124,12 @@ namespace Emby.Server.Implementations
     /// </summary>
     public abstract class ApplicationHost : IServerApplicationHost, IDisposable
     {
+        /// <summary>
+        /// The secret the startup read of the quick connect store looks for. No flow ever mints it, so the
+        /// read is always a miss and only its reachability is being asked about.
+        /// </summary>
+        private const string StartupProbeSecret = "startup-probe";
+
         /// <summary>
         /// The disposable parts.
         /// </summary>
@@ -644,6 +650,8 @@ namespace Emby.Server.Implementations
         /// <returns>A task representing the service initialization operation.</returns>
         public async Task InitializeServices(IConfiguration startupConfig)
         {
+            await ProbeQuickConnectStoreAsync().ConfigureAwait(false);
+
             var localizationManager = (LocalizationManager)Resolve<ILocalizationManager>();
             await localizationManager.LoadAll().ConfigureAwait(false);
 
@@ -652,6 +660,27 @@ namespace Emby.Server.Implementations
             FindParts();
         }
 
+        /// <summary>
+        /// Reads the quick connect store once here so a store that cannot be reached stops startup, rather
+        /// than being discovered on the first request that needs it. A read rather than a resolve because a
+        /// shared store built with <c>abortConnect=false</c> constructs without touching the network.
+        /// </summary>
+        private async Task ProbeQuickConnectStoreAsync()
+        {
+            try
+            {
+                await Resolve<IQuickConnectStore>().GetRequestBySecretAsync(StartupProbeSecret).ConfigureAwait(false);
+            }
+            catch (Exception ex)
+            {
+                Logger.LogCritical(
+                    ex,
+                    "Quick connect is configured against the shared valkey/Redis store at {Key} and it is UNREACHABLE, so the server will not start. Bring valkey up, or clear that setting to keep quick connect state on this instance alone.",
+                    TranscodeStoreOptions.RedisConnectionStringKey);
+                throw;
+            }
+        }
+
         private X509Certificate2 GetCertificate(string path, string password)
         {
             if (string.IsNullOrWhiteSpace(path))