feat(core): add audit log infrastructure (Phase 6)

- Add audit_logs database migration with indexes
- Add AuditLog struct in erp-core with builder pattern
- Support old/new value tracking, IP address, user agent

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

crates/erp-core/src/audit.rs

@@ -0,0 +1,67 @@
+use chrono::Utc;
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+/// 审计日志记录。
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AuditLog {
+    pub id: Uuid,
+    pub tenant_id: Uuid,
+    pub user_id: Option<Uuid>,
+    pub action: String,
+    pub resource_type: String,
+    pub resource_id: Option<Uuid>,
+    pub old_value: Option<serde_json::Value>,
+    pub new_value: Option<serde_json::Value>,
+    pub ip_address: Option<String>,
+    pub user_agent: Option<String>,
+    pub created_at: chrono::DateTime<Utc>,
+}
+
+impl AuditLog {
+    /// 创建一条审计日志记录。
+    pub fn new(
+        tenant_id: Uuid,
+        user_id: Option<Uuid>,
+        action: impl Into<String>,
+        resource_type: impl Into<String>,
+    ) -> Self {
+        Self {
+            id: Uuid::now_v7(),
+            tenant_id,
+            user_id,
+            action: action.into(),
+            resource_type: resource_type.into(),
+            resource_id: None,
+            old_value: None,
+            new_value: None,
+            ip_address: None,
+            user_agent: None,
+            created_at: Utc::now(),
+        }
+    }
+
+    /// 设置资源 ID。
+    pub fn with_resource_id(mut self, id: Uuid) -> Self {
+        self.resource_id = Some(id);
+        self
+    }
+
+    /// 设置变更前后的值。
+    pub fn with_changes(
+        mut self,
+        old: Option<serde_json::Value>,
+        new: Option<serde_json::Value>,
+    ) -> Self {
+        self.old_value = old;
+        self.new_value = new;
+        self
+    }
+
+    /// 设置请求来源信息。
+    pub fn with_request_info(mut self, ip: Option<String>, user_agent: Option<String>) -> Self {
+        self.ip_address = ip;
+        self.user_agent = user_agent;
+        self
+    }
+}