Fix watch mode automatic startup on server launch

Fixed a critical bug where watch mode failed to start automatically during container
initialization, despite comments in app.go:79 claiming it would start "after 2-second delay."

Root Cause:
- StartWatchModeForAllLibraries() function existed but was never invoked
- Comment in app.go claimed watch mode started automatically, but no startup code existed

Changes:
- Added "context" import to internal/router/router.go
- Added goroutine in configureRouter() that:
  * Waits 2 seconds after server initialization
  * Calls StartWatchModeForAllLibraries() to activate monitoring
  * Logs startup status or errors

This ensures watch mode begins scanning for new media files automatically when the
container starts, rather than requiring manual intervention.

Testing: Verified watch mode now activates automatically in container logs.

internal/router/router.go

@@ -10,6 +10,7 @@ import (
 	"bookhoard/internal/sync"
 	"bookhoard/templates"
 	"bytes"
+	"context"
 	"log"
 	"net/http"
 	"strings"
@@ -234,6 +235,17 @@ func RegisterRoutes(cfg *Config) *handlers.Handler {
 	// Start background tasks (queue processor and connection cleanup)
 	scannerHandler.StartBackgroundTasks()
 
+	// Start watch mode for all libraries (after 2 second delay for DB)
+	go func() {
+		time.Sleep(2 * time.Second)
+		log.Println("Starting watch mode for all libraries...")
+		if err := scannerHandler.StartWatchModeForAllLibraries(context.Background()); err != nil {
+			log.Printf("Failed to start watch mode: %v", err)
+		} else {
+			log.Println("Watch mode started successfully")
+		}
+	}()
+
 	// Register progress routes with actual handler
 	registerProgressRoutes(cfg, scannerHandler)