Fix scheduler goroutine WaitGroup leak causing shutdown deadlock

Critical bug fix: The scheduler's runSettingsChecker() goroutine was
started but never marked as complete in the WaitGroup, causing
scheduler.Stop() to hang indefinitely waiting for wg.Wait().

Changes:
- Add defer s.wg.Done() call in scheduler.Start() goroutine wrapper
- Update scheduler tests to properly call worker.Shutdown()
- Add nil check for timer.Stop() to prevent panics from nil timers
- Fix TestScheduler_StopWithActiveTimers to use proper shutdown sequence

Impact:
- Fixes test hanging issue in `make test` command
- Enables graceful shutdown of scheduler in production
- Prevents goroutine leaks in long-running applications
- All unit tests now complete successfully

Root cause: WaitGroup.Add(1) was called but Done() was never called,
creating an imbalance that caused wg.Wait() to block forever.

internal/services/scheduler.go

@@ -66,7 +66,10 @@ func NewScheduler(worker *Worker, db Database) *Scheduler {
 
 func (s *Scheduler) Start() {
 	s.wg.Add(1)
-	go s.runSettingsChecker()
+	go func() {
+		defer s.wg.Done()
+		s.runSettingsChecker()
+	}()
 }
 
 func (s *Scheduler) Stop() {
@@ -74,7 +77,9 @@ func (s *Scheduler) Stop() {
 
 	s.mu.Lock()
 	for _, timer := range s.timers {
-		timer.Stop()
+		if timer != nil {
+			timer.Stop()
+		}
 	}
 	s.timers = make(map[string]*time.Timer)
 	s.mu.Unlock()