feat(core): 审计日志哈希链 — prev_hash + record_hash + 完整性验证

- 迁移 087: audit_logs 表添加 prev_hash/record_hash 列 + 索引
- audit_service::record() 写入时查询前一条 record_hash 作为 prev_hash
- SHA256(id+action+resource_type+resource_id+created_at+prev_hash) 计算 record_hash
- verify_hash_chain() 验证链完整性，返回 (总记录数, 断链数)

crates/erp-core/src/entity/audit_log.rs

@@ -17,6 +17,10 @@ pub struct Model {
     pub ip_address: Option<String>,
     pub user_agent: Option<String>,
     pub created_at: DateTimeUtc,
+    /// 哈希链 — 前一条记录的 record_hash
+    pub prev_hash: Option<String>,
+    /// 当前记录的哈希 SHA256(id + action + resource_type + resource_id + created_at + prev_hash)
+    pub record_hash: Option<String>,
 }
 
 #[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]