feat: add router package structure for route organization

Create internal/router/ package to organize route registration:
- router.go: Main router setup and configuration
- auth.go: Authentication routes (login, register, profile, etc.)
- docs.go: Documentation routes
- frontend.go: Frontend SSR routes (/, /login, /admin, etc.)
- helpers.go: Helper functions for template rendering

This is the first step in refactoring 858-line main.go into
a more maintainable structure following Go best practices.

Routes themselves have NOT changed - only organization.

internal/router/router.go

@@ -0,0 +1,125 @@
+package router
+
+import (
+	"bookhoard/internal/config"
+	"bookhoard/internal/database"
+	"bookhoard/internal/handlers"
+	"bookhoard/internal/middleware"
+	ratelimit "bookhoard/internal/middleware"
+	"bookhoard/internal/sync"
+	"log"
+	"time"
+
+	"github.com/go-playground/validator/v10"
+	"github.com/labstack/echo/v4"
+	echomiddleware "github.com/labstack/echo/v4/middleware"
+)
+
+// CustomValidator wraps the go-playground validator
+type CustomValidator struct {
+	validator *validator.Validate
+}
+
+func (cv *CustomValidator) Validate(i interface{}) error {
+	return cv.validator.Struct(i)
+}
+
+// Config holds all dependencies needed for route registration
+type Config struct {
+	Echo                 *echo.Echo
+	Queries              *database.Queries
+	Cfg                  *config.Config
+	DBPool               interface{} // pgxpool.Pool interface
+	AuthHandler          *handlers.AuthHandler
+	LibraryHandler       *handlers.LibraryHandler
+	DeviceHandler        *handlers.DeviceHandler
+	KOReaderHandler      *handlers.KOReaderHandler
+	WSHandler            *handlers.WSHandler
+	ConflictHandler      *handlers.ConflictHandler
+	AnalyticsHandler     *handlers.AnalyticsHandler
+	QueueHandler         *handlers.QueueHandler
+	CollectionHandler    *handlers.CollectionHandler
+	OPDSHandler          *handlers.OPDSHandler
+	ConnManager          *sync.ConnectionManager
+	QueueProcessor       *sync.SyncQueueProcessor
+	DeviceAuthMiddleware *middleware.DeviceAuthMiddleware
+	LoginTracker         *ratelimit.LoginAttemptTracker
+}
+
+// RegisterRoutes registers all application routes
+func RegisterRoutes(cfg *Config) {
+	e := cfg.Echo
+
+	// Set up validator
+	v := validator.New()
+	if err := ratelimit.RegisterPasswordValidation(v); err != nil {
+		log.Fatal("Failed to register password validator:", err)
+	}
+	e.Validator = &CustomValidator{validator: v}
+
+	// Global middleware
+	e.Use(echomiddleware.Logger())
+	e.Use(echomiddleware.Recover())
+	e.Use(echomiddleware.CORS())
+	e.Use(ratelimit.RequestTracingMiddleware(cfg.Cfg))
+
+	// Rate limiter
+	rateLimiterConfig := ratelimit.RateLimiterConfig{
+		Enabled:           cfg.Cfg.RateLimitEnabled,
+		RequestsPerMinute: cfg.Cfg.RequestsPerMinute,
+		CleanupInterval:   5 * time.Minute,
+	}
+	rateLimiter := ratelimit.NewRateLimiter(rateLimiterConfig)
+	rateLimitMiddleware := ratelimit.RateLimiterMiddleware(rateLimiter)
+
+	// Register route groups
+	registerAuthRoutes(cfg, rateLimitMiddleware)
+	registerLibraryRoutes(cfg)
+	registerDeviceRoutes(cfg)
+	registerSyncRoutes(cfg)
+	registerMediaRoutes(cfg)
+	registerConflictRoutes(cfg)
+	registerAnalyticsRoutes(cfg)
+	registerQueueRoutes(cfg)
+	registerOPDSRoutes(cfg)
+	registerWebSocketRoutes(cfg)
+	registerFrontendRoutes(cfg)
+	registerDocumentationRoutes(cfg)
+}
+
+// Stub functions - will be implemented incrementally
+func registerLibraryRoutes(cfg *Config) {
+	// TODO: Implement in library.go
+}
+
+func registerDeviceRoutes(cfg *Config) {
+	// TODO: Implement in device.go
+}
+
+func registerSyncRoutes(cfg *Config) {
+	// TODO: Implement in sync.go
+}
+
+func registerMediaRoutes(cfg *Config) {
+	// TODO: Implement in media.go
+}
+
+func registerConflictRoutes(cfg *Config) {
+	// TODO: Implement in conflicts.go
+}
+
+func registerAnalyticsRoutes(cfg *Config) {
+	// TODO: Implement in analytics.go
+}
+
+func registerQueueRoutes(cfg *Config) {
+	// TODO: Implement in queue.go
+}
+
+func registerOPDSRoutes(cfg *Config) {
+	// TODO: Implement in opds.go
+}
+
+func registerWebSocketRoutes(cfg *Config) {
+	// TODO: Implement in websocket.go
+}