feat: 支持先绑定设备后关联儿童档案

- 绑定模型新增 owner_user_id，支持 child_id 为空\n- 新增绑定后设置儿童接口 /bindings/{device_id}/child\n- 前端绑定页改为先绑设备后补充儿童\n- 补充数据库迁移脚本与 DAO 测试用例

mini-program/database/sql/004_device_bindings_owner_user_migration.sql

@@ -0,0 +1,46 @@
+-- Migration: support "bind device first, then set child profile"
+-- Target: MySQL 8.x
+
+-- 1) Add owner_user_id on device_bindings.
+ALTER TABLE device_bindings
+    ADD COLUMN owner_user_id BIGINT NULL AFTER device_id;
+
+-- 2) Backfill owner_user_id from active parent-child relation (prefer primary relation).
+UPDATE device_bindings b
+SET owner_user_id = (
+    SELECT r.user_id
+    FROM parent_child_relations r
+    WHERE r.child_id = b.child_id
+      AND r.status = 1
+    ORDER BY r.is_primary DESC, r.id ASC
+    LIMIT 1
+)
+WHERE b.owner_user_id IS NULL;
+
+-- 3) Fallback backfill from latest bind history.
+UPDATE device_bindings b
+SET owner_user_id = (
+    SELECT h.bound_by_user_id
+    FROM device_bind_history h
+    WHERE h.device_id = b.device_id
+    ORDER BY h.bound_at DESC, h.id DESC
+    LIMIT 1
+)
+WHERE b.owner_user_id IS NULL;
+
+-- 4) Ensure no NULL owner before setting NOT NULL.
+-- If this query returns rows, handle manually before continuing:
+-- SELECT id, device_id, child_id FROM device_bindings WHERE owner_user_id IS NULL;
+
+-- 5) Make schema changes for new flow.
+ALTER TABLE device_bindings
+    MODIFY COLUMN owner_user_id BIGINT NOT NULL;
+
+ALTER TABLE device_bindings
+    MODIFY COLUMN child_id BIGINT NULL;
+
+ALTER TABLE device_bindings
+    ADD INDEX idx_device_bindings_owner_user_id (owner_user_id);
+
+ALTER TABLE device_bind_history
+    MODIFY COLUMN child_id BIGINT NULL;