feat: 完善模型管理功能

- 新增模型 API 路由，支持 CRUD 和测试连接
- 支持 MiniMax、GLM、OpenAI Compatible 三种供应商
- 添加连接状态持久化 (untested/connected/disconnected)
- 修复 CORS 和数据库模型兼容性问题
- 前端 UI 优化：供应商默认 API 地址自动填充

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/models.py

@@ -135,12 +135,13 @@ class ModelConfig(Base, UUIDMixin, TimestampMixin):
     """Model configuration for LLM providers"""
     __tablename__ = "model_configs"
 
-    project_id = Column(UUID(as_uuid=True), ForeignKey("projects.id", ondelete="CASCADE"), nullable=False)
-    provider = Column(String(50), nullable=False)  # openai, anthropic, ollama, custom
+    project_id = Column(UUID(as_uuid=True), ForeignKey("projects.id", ondelete="CASCADE"), nullable=True)
+    provider = Column(String(50), nullable=False)  # minimax, glm, openai
     model_name = Column(String(100))
     api_key = Column(String(500))
     api_base = Column(String(500))
     is_default = Column(String(10), default="false")
+    connection_status = Column(String(20), default="untested")  # untested, connected, disconnected
 
     # Relationships
     project = relationship("Project", back_populates="model_configs")