fix: crash on launch from BGTask handler @MainActor isolation violation

BGTaskScheduler.register(using: nil) invokes the handler on a background
queue, but the closure captured @MainActor-isolated self. Swift 6 runtime
enforces this with dispatch_assert_queue which crashed on Thread 4.

Fix: pass DispatchQueue.main as the handler queue so the callback runs
on the main queue, satisfying @MainActor isolation. Also fix expiration
handlers to capture a local Logger copy instead of accessing self.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SportsTime/Core/Services/BackgroundSyncManager.swift

@@ -60,9 +60,12 @@ final class BackgroundSyncManager {
     /// Must be called early in app lifecycle, before `applicationDidFinishLaunching` returns.
     func registerTasks() {
         // Register refresh task (for periodic CloudKit sync)
+        // using: DispatchQueue.main ensures the handler runs on the main queue,
+        // satisfying @MainActor isolation (using: nil invokes on a background queue,
+        // which crashes with dispatch_assert_queue_fail in Swift 6)
         BGTaskScheduler.shared.register(
             forTaskWithIdentifier: Self.refreshTaskIdentifier,
-            using: nil
+            using: DispatchQueue.main
         ) { [weak self] task in
             guard let task = task as? BGAppRefreshTask else { return }
             Task { @MainActor in
@@ -73,7 +76,7 @@ final class BackgroundSyncManager {
         // Register processing task (for overnight heavy sync/cleanup)
         BGTaskScheduler.shared.register(
             forTaskWithIdentifier: Self.processingTaskIdentifier,
-            using: nil
+            using: DispatchQueue.main
         ) { [weak self] task in
             guard let task = task as? BGProcessingTask else { return }
             Task { @MainActor in
@@ -154,8 +157,10 @@ final class BackgroundSyncManager {
         currentCancellationToken = cancellationToken
 
         // Set up expiration handler - cancel sync gracefully
-        task.expirationHandler = { [weak self] in
-            self?.logger.warning("Background refresh task expiring - cancelling sync")
+        // Note: expirationHandler runs on an arbitrary queue, so avoid accessing @MainActor self
+        let expirationLogger = logger
+        task.expirationHandler = {
+            expirationLogger.warning("Background refresh task expiring - cancelling sync")
             cancellationToken.cancel()
         }
 
@@ -196,8 +201,9 @@ final class BackgroundSyncManager {
         currentCancellationToken = cancellationToken
 
         // Set up expiration handler - cancel sync gracefully
-        task.expirationHandler = { [weak self] in
-            self?.logger.warning("Background processing task expiring - cancelling sync")
+        let processingLogger = logger
+        task.expirationHandler = {
+            processingLogger.warning("Background processing task expiring - cancelling sync")
             cancellationToken.cancel()
         }