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

tests/Jellyfin.Server.Tests/HighAvailability/TranscodeStoreConnectivityProbeTests.cs

@@ -0,0 +1,75 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Emby.Server.Implementations.MediaEncoding;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Moq;
+using StackExchange.Redis;
+using Xunit;
+
+namespace Jellyfin.Server.Tests.HighAvailability;
+
+/// <summary>
+/// A Redis store that cannot be reached degrades silently: the client is configured not to abort the
+/// connection and every call site swallows failures. The probe is the only startup signal, so both of
+/// its outcomes are pinned here.
+/// </summary>
+public sealed class TranscodeStoreConnectivityProbeTests
+{
+    [Fact]
+    public async Task StartAsync_Should_Log_Information_When_Reachable()
+    {
+        var database = new Mock<IDatabase>();
+        database.Setup(db => db.PingAsync(It.IsAny<CommandFlags>())).ReturnsAsync(TimeSpan.FromMilliseconds(3));
+
+        var (logger, probe) = CreateProbe(database.Object);
+
+        await probe.StartAsync(CancellationToken.None);
+
+        Assert.True(logger.HasEntry(LogLevel.Information, "reachable"));
+        Assert.DoesNotContain(logger.Entries, entry => entry.Level >= LogLevel.Warning);
+    }
+
+    [Fact]
+    public async Task StartAsync_Should_Log_Error_When_Unreachable()
+    {
+        var database = new Mock<IDatabase>();
+        database.Setup(db => db.PingAsync(It.IsAny<CommandFlags>()))
+            .ThrowsAsync(new RedisConnectionException(ConnectionFailureType.UnableToConnect, "no route to host"));
+
+        var (logger, probe) = CreateProbe(database.Object);
+
+        await probe.StartAsync(CancellationToken.None);
+
+        Assert.True(logger.HasEntry(LogLevel.Error, "UNREACHABLE"));
+    }
+
+    [Fact]
+    public async Task StartAsync_Should_Log_Error_Instead_Of_Aborting_Startup()
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<IConnectionMultiplexer>(_ => throw new RedisConnectionException(ConnectionFailureType.UnableToConnect, "no route to host"));
+        using var provider = services.BuildServiceProvider();
+
+        var logger = new RecordingLogger<TranscodeStoreConnectivityProbe>();
+        var probe = new TranscodeStoreConnectivityProbe(provider, logger);
+
+        await probe.StartAsync(CancellationToken.None);
+
+        Assert.True(logger.HasEntry(LogLevel.Error, "UNREACHABLE"));
+    }
+
+    private static (RecordingLogger<TranscodeStoreConnectivityProbe> Logger, TranscodeStoreConnectivityProbe Probe) CreateProbe(IDatabase database)
+    {
+        var multiplexer = new Mock<IConnectionMultiplexer>();
+        multiplexer.Setup(redis => redis.GetDatabase(It.IsAny<int>(), It.IsAny<object>())).Returns(database);
+
+        var services = new ServiceCollection();
+        services.AddSingleton(multiplexer.Object);
+        var provider = services.BuildServiceProvider();
+
+        var logger = new RecordingLogger<TranscodeStoreConnectivityProbe>();
+        return (logger, new TranscodeStoreConnectivityProbe(provider, logger));
+    }
+}