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

docs/FORK-DIFF.md

@@ -37,6 +37,9 @@ auth or plugin logic is rewritten.
 | `MediaBrowser.Controller/MediaEncoding/TranscodeStoreOptions.cs` | `RedisConnectionString`, `LeaseDurationSeconds` and `SessionRetentionSeconds` |
 | `MediaBrowser.Controller/MediaEncoding/NullTranscodeSessionStore.cs` | No-op store used when no Redis connection is configured |
 | `Emby.Server.Implementations/MediaEncoding/RedisTranscodeSessionStore.cs` | Redis store; sessions under `jellyfin:transcode:{playSessionId}`, key TTL is the retention window so an orphaned session outlives its lease |
+| `Emby.Server.Implementations/MediaEncoding/TranscodeStoreConnectivityProbe.cs` | Startup ping; an unreachable configured store is logged at `Error` instead of failing open silently |
+| `Jellyfin.Server/Extensions/TranscodeStoreServiceCollectionExtensions.cs` | Store selection, logged at `Information` so the active store is visible at startup |
+| `Jellyfin.Server/Extensions/ConfigurationBuilderExtensions.cs` | Reads bare `Jellyfin__*` environment variables into the `Jellyfin:*` configuration root |
 
 Lease takeover and renewal each run as a single Lua script, so concurrent pods cannot both
 claim an expired lease and a renewal cannot revert a takeover. The expiry is stored as unix