feat(ai): Day 7 — 会话持久化 Entity + Service

- 新增 3 个 SeaORM Entity: ai_chat_session / ai_chat_message / ai_tool_call_log
- ChatSessionService: create / list / get / close / rename
- ChatMessageService: save_message / list_messages / save_tool_call_log
- 参数封装为 SaveMessageParams / SaveToolCallLogParams 避免 clippy too_many_arguments
- AiState 注册 chat_session + chat_message 服务
- erp-server main.rs 初始化注入

crates/erp-ai/src/entity/mod.rs

@@ -1,5 +1,7 @@
 pub mod ai_analysis;
 pub mod ai_analysis_queue;
+pub mod ai_chat_message;
+pub mod ai_chat_session;
 pub mod ai_feature_flags;
 pub mod ai_knowledge_guides;
 pub mod ai_knowledge_references;
@@ -9,6 +11,7 @@ pub mod ai_risk_threshold;
 pub mod ai_suggestion;
 pub mod ai_suggestion_feedback;
 pub mod ai_tenant_config;
+pub mod ai_tool_call_log;
 pub mod ai_usage;
 pub mod ai_usage_daily;
 pub mod copilot_chat_logs;