refactor: 強化 Quiz Option 生成機制防止重複詞彙

- 加強 AI 生成後的詞彙過濾，確保不包含目標詞彙
- 改進 fallback 選項品質，使用更具挑戰性的詞彙池
- 添加詳細日誌追蹤選項生成過程（✅📚💡🤖）
- 修復資料庫重複詞彙問題，確保選項品質

測試驗證：happy -> efficient, essential, fundamental

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/DramaLing.Api/Services/Vocabulary/Options/OptionsVocabularyService.cs

@@ -79,7 +79,7 @@ public class OptionsVocabularyService : IOptionsVocabularyService
 
             if (_cache.TryGetValue(cacheKey, out List<OptionsVocabulary>? cachedOptions) && cachedOptions != null)
             {
-                _logger.LogInformation("Using cached distractors for '{Word}': {Distractors}",
+                _logger.LogInformation("✅ Using cached distractors for '{Word}': {Distractors}",
                     targetWord, string.Join(", ", cachedOptions.Select(d => d.Word)));
                 return cachedOptions.Take(count).ToList();
             }
@@ -104,11 +104,14 @@ public class OptionsVocabularyService : IOptionsVocabularyService
                 // 快取結果
                 _cache.Set(cacheKey, selectedExisting, TimeSpan.FromHours(1));
 
-                _logger.LogInformation("Using existing distractors for '{Word}': {Distractors}",
+                _logger.LogInformation("📚 Using existing database distractors for '{Word}': {Distractors}",
                     targetWord, string.Join(", ", selectedExisting.Select(d => d.Word)));
                 return selectedExisting;
             }
 
+            _logger.LogInformation("💡 Insufficient database options ({ExistingCount}/{RequiredCount}), proceeding to AI generation for '{Word}'",
+                existingOptions.Count, count, targetWord);
+
             // 3. 使用 AI 生成新的混淆選項
             var aiGeneratedOptions = await GenerateOptionsWithAI(targetWord, cefrLevel, partOfSpeech, count);
 
@@ -122,7 +125,7 @@ public class OptionsVocabularyService : IOptionsVocabularyService
                 _cache.Set(cacheKey, aiGeneratedOptions, TimeSpan.FromHours(1));
             }
 
-            _logger.LogInformation("Successfully generated {Count} AI distractors for '{Word}': {Distractors}",
+            _logger.LogInformation("🤖 Successfully generated {Count} AI distractors for '{Word}': {Distractors}",
                 aiGeneratedOptions.Count, targetWord,
                 string.Join(", ", aiGeneratedOptions.Select(d => d.Word)));
 
@@ -247,8 +250,14 @@ Please respond with ONLY a JSON array of strings, like: [""word1"", ""word2"", "
                 return GetFallbackDistractors(targetWord, cefrLevel, partOfSpeech, count);
             }
 
-            // 轉換為 OptionsVocabulary 實體
-            var options = generatedWords.Take(count).Select(word => new OptionsVocabulary
+            // 轉換為 OptionsVocabulary 實體，並過濾掉與目標詞彙相同的詞彙
+            var targetWordLower = targetWord.ToLower();
+            var filteredWords = generatedWords
+                .Where(word => !string.IsNullOrWhiteSpace(word) &&
+                              word.Trim().ToLower() != targetWordLower)
+                .Take(count);
+
+            var options = filteredWords.Select(word => new OptionsVocabulary
             {
                 Id = Guid.NewGuid(),
                 Word = word.Trim(),
@@ -259,6 +268,9 @@ Please respond with ONLY a JSON array of strings, like: [""word1"", ""word2"", "
                 UpdatedAt = DateTime.UtcNow
             }).ToList();
 
+            _logger.LogInformation("Filtered AI generated words from {OriginalCount} to {FilteredCount} options for '{TargetWord}'",
+                generatedWords.Length, options.Count, targetWord);
+
             // 計算字數長度
             foreach (var option in options)
             {
@@ -283,20 +295,27 @@ Please respond with ONLY a JSON array of strings, like: [""word1"", ""word2"", "
         string partOfSpeech,
         int count)
     {
-        // 根據詞性提供不同的固定選項
+        // 根據詞性提供不同的固定選項（避免常見目標詞彙）
         var fallbackWords = partOfSpeech.ToLower() switch
         {
-            "verb" => new[] { "create", "destroy", "change", "develop", "improve", "reduce" },
-            "noun" => new[] { "example", "result", "problem", "method", "system", "process" },
-            "adjective" => new[] { "important", "different", "special", "general", "common", "simple" },
-            "adverb" => new[] { "quickly", "carefully", "easily", "slowly", "clearly", "directly" },
-            _ => new[] { "option", "choice", "answer", "question", "test", "study" }
+            "verb" => new[] { "accomplish", "construct", "transform", "establish", "generate", "implement", "maintain", "organize", "validate", "coordinate" },
+            "noun" => new[] { "solution", "approach", "framework", "component", "structure", "mechanism", "principle", "strategy", "technique", "procedure" },
+            "adjective" => new[] { "efficient", "comprehensive", "innovative", "sophisticated", "fundamental", "essential", "remarkable", "outstanding", "significant", "substantial" },
+            "adverb" => new[] { "thoroughly", "efficiently", "precisely", "consistently", "systematically", "effectively", "accurately", "appropriately", "specifically", "particularly" },
+            _ => new[] { "element", "aspect", "feature", "component", "factor", "criteria", "parameter", "attribute", "characteristic", "specification" }
         };
 
         var targetWordLower = targetWord.ToLower();
-        return fallbackWords
+        var selectedWords = fallbackWords
             .Where(word => word.ToLower() != targetWordLower)
-            .Take(count)
+            .Take(count * 2)  // 取更多選項用於隨機選擇
+            .OrderBy(x => Guid.NewGuid())  // 隨機排序
+            .Take(count);
+
+        _logger.LogInformation("Using fallback distractors for '{TargetWord}': {FallbackWords}",
+            targetWord, string.Join(", ", selectedWords));
+
+        return selectedWords
             .Select(word => new OptionsVocabulary
             {
                 Id = Guid.NewGuid(),