docs: Add package documentation for handlers and services

Add Go package documentation comments to clarify the purpose and scope of:

- internal/handlers/: HTTP request/response handlers for authentication,
  libraries, media items, reading, collections, dashboards, devices,
  analytics, and system features

- internal/services/: Core business logic layer including media scanning,
  library management, search, analytics, and conversion services

These doc comments improve code discoverability and help developers understand
the architectural separation between HTTP handling (handlers) and business
logic (services).

internal/handlers/auth.go

@@ -1,3 +1,6 @@
+// Package handlers provides HTTP request/response handlers for the bookhoard application.
+// It includes handlers for authentication, libraries, media items, reading, collections,
+// dashboards, devices, analytics, and various system features.
 package handlers
 
 import (