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

README.md

@@ -72,7 +72,7 @@ dotnet run --project Jellyfin.Server/Jellyfin.Server.csproj -- \
 
 ### HA mode with Redis
 
-Set the `Jellyfin:TranscodeStore:RedisConnectionString` configuration key. You can pass it as an environment variable, a `DOTNET_` prefixed env var, or in a JSON config file.
+Set the `Jellyfin:TranscodeStore:RedisConnectionString` configuration key. You can pass it as a `Jellyfin__TranscodeStore__RedisConnectionString` environment variable, as the equivalent `JELLYFIN_` prefixed variable, or in a JSON config file.
 
 **Environment variable:**
 
@@ -98,7 +98,14 @@ dotnet run --project Jellyfin.Server/Jellyfin.Server.csproj -- \
 }
 ```
 
-When `RedisConnectionString` is set, `RedisTranscodeSessionStore` is registered in DI. If the Redis connection fails at startup, the server throws and refuses to start — this is intentional so you don't silently fall back to broken HA behavior.
+The selected store is logged at startup, so HA transcoding is never on or off without a signal:
+
+```
+Transcode session store: RedisTranscodeSessionStore on valkey:6379.
+Redis transcode session store is reachable (2ms round trip). HA transcode takeover is active.
+```
+
+Without a connection string the line reads `Transcode session store: NullTranscodeSessionStore`. A configured but unreachable store is logged at `Error`; the server keeps serving with per-instance sessions rather than refusing to start.
 
 ---