fix(ha): read transcode store config from the variable form deployments set

The startup configuration only reads JELLYFIN_ prefixed environment
variables, so the bare Jellyfin__TranscodeStore__* form used by the chart,
the manifests and the README is dropped and the Redis store is never
registered. Nothing logs the selected store, so the fallback is invisible.

- Read bare Jellyfin__* variables into the Jellyfin:* configuration root
- Keep an explicit JELLYFIN_ variable winning over the bare form
- Log the selected transcode session store at startup
- Ping Redis once at startup and log an unreachable store at Error
- Log endpoints only, never the connection string

Jellyfin.Server/Extensions/ConfigurationBuilderExtensions.cs

@@ -0,0 +1,51 @@
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.Extensions.Configuration;
+
+namespace Jellyfin.Server.Extensions;
+
+/// <summary>
+/// Extensions for building the application configuration.
+/// </summary>
+public static class ConfigurationBuilderExtensions
+{
+    /// <summary>
+    /// The environment variable prefix that maps onto this fork's own <c>Jellyfin:*</c> configuration
+    /// keys, for example <c>Jellyfin__TranscodeStore__RedisConnectionString</c>.
+    /// </summary>
+    public const string JellyfinSectionEnvironmentPrefix = "Jellyfin__";
+
+    /// <summary>
+    /// The configuration section root this fork keeps its own settings under.
+    /// </summary>
+    public const string JellyfinSectionRoot = "Jellyfin";
+
+    /// <summary>
+    /// Adds environment variables named <c>Jellyfin__Section__Key</c> as the configuration keys
+    /// <c>Jellyfin:Section:Key</c>.
+    /// </summary>
+    /// <remarks>
+    /// The base configuration only reads <c>JELLYFIN_</c> prefixed environment variables, so the
+    /// unprefixed form every manifest, chart and document uses would otherwise be dropped and the
+    /// feature it configures would stay off with no error.
+    /// </remarks>
+    /// <param name="builder">The configuration builder.</param>
+    /// <returns>The updated configuration builder.</returns>
+    public static IConfigurationBuilder AddJellyfinSectionEnvironmentVariables(this IConfigurationBuilder builder)
+    {
+        // Read through the framework provider so "__" to ":" normalisation and case handling match the
+        // prefixed form exactly; the prefix it strips is then put back as the section root.
+        var scoped = new ConfigurationBuilder()
+            .AddEnvironmentVariables(JellyfinSectionEnvironmentPrefix)
+            .Build();
+
+        var entries = scoped.AsEnumerable()
+            .Where(entry => entry.Value is not null)
+            .Select(entry => new KeyValuePair<string, string?>(
+                ConfigurationPath.Combine(JellyfinSectionRoot, entry.Key),
+                entry.Value))
+            .ToList();
+
+        return builder.AddInMemoryCollection(entries);
+    }
+}