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/ai_chat_message.rs

@@ -0,0 +1,41 @@
+use sea_orm::entity::prelude::*;
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel, Serialize, Deserialize)]
+#[sea_orm(table_name = "ai_chat_messages")]
+pub struct Model {
+    #[sea_orm(primary_key, auto_increment = false)]
+    pub id: Uuid,
+    pub tenant_id: Uuid,
+    pub session_id: Uuid,
+    pub role: String,
+    pub content: Option<String>,
+    pub tool_calls: Option<serde_json::Value>,
+    pub tool_call_id: Option<String>,
+    pub token_count: Option<i32>,
+    pub created_at: DateTimeUtc,
+    pub updated_at: DateTimeUtc,
+    pub created_by: Option<Uuid>,
+    pub updated_by: Option<Uuid>,
+    pub deleted_at: Option<DateTimeUtc>,
+    #[sea_orm(default_value = 1)]
+    pub version_lock: i32,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {
+    #[sea_orm(
+        belongs_to = "super::ai_chat_session::Entity",
+        from = "Column::SessionId",
+        to = "super::ai_chat_session::Column::Id"
+    )]
+    Session,
+}
+
+impl Related<super::ai_chat_session::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::Session.def()
+    }
+}
+
+impl ActiveModelBehavior for ActiveModel {}