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

docs/FORK-DIFF.md

@@ -60,7 +60,7 @@ return 1
 | File | Purpose |
 |------|---------|
 | `MediaBrowser.Controller/ScheduledTasks/IScanLeaderLease.cs` | DI contract for the leader lease |
-| `MediaBrowser.Controller/ScheduledTasks/ScanLeaderOptions.cs` | `Enabled`, `LeaseDurationSeconds`, `GatedTaskKeys` |
+| `MediaBrowser.Controller/ScheduledTasks/ScanLeaderOptions.cs` | `Enabled` (defaults to on when Redis is configured), `LeaseDurationSeconds`, `GatedTaskKeys` |
 | `MediaBrowser.Controller/ScheduledTasks/NullScanLeaderLease.cs` | Always-leader default, preserving single-instance behaviour |
 | `Emby.Server.Implementations/ScheduledTasks/RedisScanLeaderLease.cs` | Redis TTL lease; an unreachable Redis is treated as holding the lease |
 
@@ -69,6 +69,17 @@ Gated by default: `RefreshLibrary`, `RefreshPeople`, `RefreshChapterImages`,
 `CleanupUserDataTask`, `OptimizeDatabaseTask`. Only timer-driven runs are gated;
 manual and API-triggered runs always execute locally.
 
+Gating is on whenever `Jellyfin:TranscodeStore:RedisConnectionString` is set, because that is only
+set for a multi-instance deployment. Set `Jellyfin:ScanLeader:Enabled=false` to opt out.
+Startup logs which way it went:
+
+```
+Scan-leader gating is active: timer-driven library tasks run only on the instance holding the Redis scan-leader lease.
+Scan-leader gating is off: timer-driven library tasks run on every instance.
+```
+
+`Enabled=true` with no Redis connection string logs a warning, because gating cannot run.
+
 ### PostgreSQL provider
 
 `src/Jellyfin.Database/Jellyfin.Database.Providers.PostgreSQL/` is an EF Core