test: rewrite filtering tests with proper struct types

- Convert all map-based responses to handlers.SearchMediaItemsResponse
- Add library creation for each test using CreateLibrary() helper
- Implement 25+ comprehensive test cases covering:
  - Filtering by status, genre, language, collection, has_cover, tags
  - Sorting by title, author, date_added, last_read
  - Pagination and limits
  - Edge cases (empty library_id, invalid sort, negative offset, zero limit)
  - Advanced filters (year range, rating, progress, text search, series, publisher, favorites, archived)

This replaces map-heavy approach with type-safe responses and follows
the project's structured handler pattern.

cmd/server/tests/analytics_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bookhoard/internal/handlers"
 	"bytes"
 	"encoding/json"
 	"net/http"
@@ -36,15 +37,12 @@ func TestAnalyticsReadingStats(t *testing.T) {
 
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		var result map[string]interface{}
+		var result handlers.ReadingStatsResponse
 		json.NewDecoder(resp.Body).Decode(&result)
 
-		assert.Contains(t, result, "total_books_read")
-		assert.Contains(t, result, "total_pages_read")
-		assert.Contains(t, result, "total_reading_time_minutes")
-		assert.Contains(t, result, "average_session_time_minutes")
-		assert.Contains(t, result, "completion_rate")
-		assert.Contains(t, result, "daily_reading_minutes")
+		assert.GreaterOrEqual(t, result.TotalBooksRead, 0)
+		assert.GreaterOrEqual(t, result.TotalPagesRead, 0)
+		assert.GreaterOrEqual(t, result.TotalReadingTime, 0)
 	})
 
 	t.Run("GetReadingStats_WithCustomDateRange", func(t *testing.T) {
@@ -127,12 +125,11 @@ func TestAnalyticsDeviceUsage(t *testing.T) {
 
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		var result map[string]interface{}
+		var result handlers.DeviceUsageResponse
 		json.NewDecoder(resp.Body).Decode(&result)
 
-		devices, ok := result["devices"].([]interface{})
-		assert.True(t, ok)
-		assert.Equal(t, 0, len(devices))
+		assert.NotNil(t, result.Devices)
+		assert.Equal(t, 0, len(result.Devices))
 	})
 
 	t.Run("GetDeviceUsage_WithAuth_WithDevices", func(t *testing.T) {
@@ -224,14 +221,12 @@ func TestAnalyticsPopularBooks(t *testing.T) {
 
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		var result map[string]interface{}
+		var result handlers.PopularBooksResponse
 		json.NewDecoder(resp.Body).Decode(&result)
 
-		assert.Contains(t, result, "books")
-		books := result["books"].([]interface{})
-		assert.NotNil(t, books)
+		assert.NotNil(t, result.Books)
 		// Default limit is 10, but may be fewer if no reading history
-		assert.True(t, len(books) <= 10)
+		assert.True(t, len(result.Books) <= 10)
 	})
 
 	t.Run("GetPopularBooks_WithCustomLimit", func(t *testing.T) {