fix: synonyms字段从KeywordGroup移到Keyword模型

- synonyms字段实际在keywords表，但模型错放在KeywordGroup
- 导致关键词API 500错误和关键词匹配失败

Author: chu0119

backend/app/models/keyword.py

@@ -48,9 +48,6 @@ class KeywordGroup(Base):
     total_keywords = Column(Integer, default=0, comment="关键词数量")
     total_matches = Column(Integer, default=0, comment="总匹配次数")
 
-    # 同义词扩展
-    synonyms = Column(JSON, comment="同义词列表")
-
     # 元数据
     created_at = Column(DateTime, server_default=func.now(), comment="创建时间")
     updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), comment="更新时间")
@@ -81,6 +78,9 @@ class Keyword(Base):
     match_count = Column(Integer, default=0, comment="匹配次数")
     last_matched_at = Column(DateTime, comment="最后匹配时间")
 
+    # 同义词扩展
+    synonyms = Column(JSON, comment="同义词列表")
+
     # 状态
     is_active = Column(Boolean, default=True, comment="是否激活")