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.

internal/router/router.go

@@ -52,6 +52,7 @@ type Config struct {
 	AnalyticsHandler      *handlers.AnalyticsHandler
 	QueueHandler          *handlers.QueueHandler
 	CollectionHandler     *handlers.CollectionHandler
+	Worker                *services.Worker
 	DashboardHandler      *handlers.DashboardHandler
 	DashboardService      *services.DashboardService
 	OPDSHandler           *handlers.OPDSHandler