feat: refactor health check endpoint with real-time worker status

Extract health check logic into GetHealth method on Config struct and
integrate with Worker service for accurate scan status reporting.

Changes:
- Move health check handler from inline function to Config.GetHealth()
- Add Worker field to Config struct for dependency injection
- Wire Worker into main server dependencies
- Report actual scan_in_progress status using Worker.HasActiveScans()
- Report actual active_jobs count using Worker.GetActiveJobCount()

This provides more accurate health monitoring by checking the real state
of background jobs rather than returning static placeholder values.

cmd/server/main.go

@@ -175,6 +175,7 @@ func main() {
 		DashboardHandler:      dashboardHandler,
 		DashboardService:      dashboardService,
 		OPDSHandler:           opdsHandler,
+		Worker:                worker,
 		SystemSettingsHandler: systemSettingsHandler,
 		ConnManager:           connManager,
 		QueueProcessor:        queueProcessor,