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-server/migration/src/lib.rs

@@ -86,6 +86,7 @@ mod m20260427_000083_create_follow_up_template;
 mod m20260427_000084_domain_events_cleanup;
 mod m20260427_000085_processed_events;
 mod m20260427_000086_enable_rls_all_tables;
+mod m20260427_000087_audit_logs_hash_chain;
 
 pub struct Migrator;
 
@@ -179,6 +180,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260427_000084_domain_events_cleanup::Migration),
             Box::new(m20260427_000085_processed_events::Migration),
             Box::new(m20260427_000086_enable_rls_all_tables::Migration),
+            Box::new(m20260427_000087_audit_logs_hash_chain::Migration),
         ]
     }
 }