refactor(settings): 统一语言设置，合并 interface_language 和 ai_language

- 后端 Schema 将 interface_language 和 ai_language 合并为 language
- 前端设置界面只保留一个语言选项
- AI 回复语言统一使用 language 设置
- 更新协议文档
- 新增数据库迁移脚本

backend/tests/unit/test_agentscope_prompts.py

@@ -5,10 +5,10 @@ from core.agentscope.prompts.system_prompt import build_system_prompt
 from schemas.agent.system_agent import AgentType, SystemAgentLLMConfig
 
 
-def test_system_prompt_enforces_ai_language_en() -> None:
+def test_system_prompt_enforces_language_en() -> None:
     prompt = build_system_prompt(
         agent_type=AgentType.WORKER,
-        ai_language="en-US",
+        language="en-US",
         llm_config=SystemAgentLLMConfig(),
     )
 
@@ -17,10 +17,10 @@ def test_system_prompt_enforces_ai_language_en() -> None:
     assert "<!-- OUTPUT_START -->" in prompt
 
 
-def test_system_prompt_enforces_ai_language_zh_cn() -> None:
+def test_system_prompt_enforces_language_zh_cn() -> None:
     prompt = build_system_prompt(
         agent_type=AgentType.WORKER,
-        ai_language="zh-CN",
+        language="zh-CN",
         llm_config=SystemAgentLLMConfig(),
     )
 
@@ -31,7 +31,7 @@ def test_system_prompt_enforces_ai_language_zh_cn() -> None:
 def test_system_prompt_safety_restricts_to_divination() -> None:
     prompt = build_system_prompt(
         agent_type=AgentType.WORKER,
-        ai_language="zh-CN",
+        language="zh-CN",
         llm_config=SystemAgentLLMConfig(),
     )
 
@@ -41,7 +41,7 @@ def test_system_prompt_safety_restricts_to_divination() -> None:
 def test_system_prompt_does_not_contain_env_section() -> None:
     prompt = build_system_prompt(
         agent_type=AgentType.WORKER,
-        ai_language="zh-CN",
+        language="zh-CN",
         llm_config=SystemAgentLLMConfig(),
     )
 
@@ -65,7 +65,7 @@ def test_agent_prompt_keeps_only_identity_and_domain_flow() -> None:
 def test_system_prompt_sections_are_not_duplicated() -> None:
     prompt = build_system_prompt(
         agent_type=AgentType.WORKER,
-        ai_language="zh-CN",
+        language="zh-CN",
         llm_config=SystemAgentLLMConfig(),
     )