refactor: complete router package migration

Major refactoring milestone - migrate all routes from main.go to internal/router/ package:

## Changes

### cmd/server/main.go
- Reduced from 858 lines to 163 lines (81% reduction)
- Removed all inline route definitions
- Added router.RegisterRoutes() call with full config
- Clean separation: setup → router registration → server start

### internal/router/ package
Created comprehensive route organization:
- router.go: Main router setup and JWT middleware
- auth.go: Authentication routes (login, register, profile, etc.)
- library.go: Library management routes
- device.go: Device registration and management
- sync.go: KOReader/Kobo sync + book matching + WebSocket
- media.go: Media download, shelves, bulk operations
- conflicts.go: Conflict resolution routes
- analytics.go: Analytics API routes
- queue.go: Sync queue management
- opds.go: OPDS feed routes
- frontend.go: SSR pages (/login, /admin, /dashboard, etc.)
- docs.go: Documentation routes
- helpers.go: Template rendering helpers

## Verification
✅ All 26 guideline checks pass
✅ Code compiles successfully
✅ Zero API behavior changes (100% compatible)
✅ Follows Go standard project layout

## Breaking Changes
None - API compatibility fully maintained

internal/router/media.go

@@ -0,0 +1,36 @@
+package router
+
+import (
+	"bookhoard/internal/handlers"
+
+	"github.com/labstack/echo-jwt/v4"
+)
+
+func registerMediaRoutes(cfg *Config) {
+	e := cfg.Echo
+
+	// JWT middleware for protected routes
+	jwtMiddleware := echojwt.WithConfig(echojwt.Config{
+		SigningKey: []byte(cfg.Cfg.JWTSecret),
+		ContextKey: "user",
+	})
+
+	protected := e.Group("/api", jwtMiddleware)
+
+	// Media item handler
+	mediaHandler := handlers.NewMediaHandler(cfg.Queries)
+
+	// Download route (public)
+	e.GET("/api/books/:uuid/download", mediaHandler.DownloadBook)
+
+	// Shelf management (protected)
+	protected.POST("/devices/:id/shelves", mediaHandler.AddToShelf)
+	protected.GET("/devices/:id/shelves", mediaHandler.GetShelf)
+	protected.DELETE("/devices/:id/shelves", mediaHandler.RemoveFromShelf)
+	protected.DELETE("/devices/:id/shelves/clear", mediaHandler.ClearShelf)
+
+	// Bulk book operations (protected)
+	books := protected.Group("/books")
+	books.POST("/bulk-delete", mediaHandler.HandleBulkDelete)
+	books.POST("/bulk-update", mediaHandler.HandleBulkUpdate)
+}