feat: 商业认证完整流程 + 通知时间过滤

商业认证:
- 申请页重写：商户名+说明字段，显示审核中/被拒/已通过状态
- 被拒绝显示原因，可重新申请
- 管理员拒绝时输入原因
- 商业核算页未认证用户点详情弹认证提示，跳转认证页面
- 删除项目仅管理员可见

通知:
- 只显示用户注册后的通知，避免角色变更后看到旧通知
- 搜索未收录通知只发管理员和高级编辑者

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/main.py

@@ -1420,11 +1420,15 @@ def get_notifications(user=Depends(get_current_user)):
     if not user["id"]:
         return []
     conn = get_db()
+    # Only show notifications created after user registration
+    user_created = conn.execute("SELECT created_at FROM users WHERE id = ?", (user["id"],)).fetchone()
+    created_at = user_created["created_at"] if user_created else "2000-01-01"
     rows = conn.execute(
         "SELECT id, title, body, is_read, created_at FROM notifications "
         "WHERE (target_user_id = ? OR (target_user_id IS NULL AND (target_role = ? OR target_role = 'all'))) "
+        "AND created_at >= ? "
         "ORDER BY is_read ASC, id DESC LIMIT 200",
-        (user["id"], user["role"])
+        (user["id"], user["role"], created_at)
     ).fetchall()
     conn.close()
     return [dict(r) for r in rows]