fix(ha): gate library tasks on the scan leader by default

Jellyfin:ScanLeader:Enabled defaults to false and nothing sets it, so
leader election never runs and every replica executes the timer-driven
library tasks concurrently - the exact behaviour the lease prevents.

- Enable gating by default when a Redis connection string is configured
- Honour an explicit Enabled setting either way
- Carry the effective decision onto the bound options the task worker reads
- Log at startup whether gating is active
- Warn when gating is enabled but no Redis connection string is configured

MediaBrowser.Controller/ScheduledTasks/ScanLeaderOptions.cs

@@ -6,9 +6,20 @@ namespace MediaBrowser.Controller.ScheduledTasks;
 /// </summary>
 public sealed class ScanLeaderOptions
 {
+    /// <summary>
+    /// The configuration section these options bind from.
+    /// </summary>
+    public const string ConfigurationSection = "Jellyfin:ScanLeader";
+
+    /// <summary>
+    /// The configuration key that overrides the default enablement.
+    /// </summary>
+    public const string EnabledKey = ConfigurationSection + ":Enabled";
+
     /// <summary>
     /// Gets or sets a value indicating whether scan-leader election is enabled. When disabled,
-    /// every instance runs its periodic tasks as before.
+    /// every instance runs its periodic tasks as before. Left unset, election is enabled whenever a
+    /// Redis connection string is configured, because that is the only deployment shape that needs it.
     /// </summary>
     public bool Enabled { get; set; }