fix(voice): composed agent 复述日程时间不再带时区偏移

[yyy-router]

Part of #410

改动

组合式 agent 序列化日程给 LLM 时只有 start_time 的 ISO 字符串，偏移随 PostgreSQL 服务器时区漂移。服务器容器默认 UTC 时，LLM 口播直接读钟面数字，把「下午六点」说成「上午十点」，固定差 8 小时。

对齐 realtime 的 _for_model：_schedule_for_llm 新增 starts_at_local 字段，用日程自身 IANA 时区（ZoneInfo(snapshot.timezone)）把存储时刻渲染成本地墙上时间。

验证

• 后端 pytest 1248 passed、49 skipped，覆盖率 97.24%（门禁 ≥95%）
• ruff check、ruff format --check、mypy strict 全绿
• 新增测试 test_serialized_snapshot_renders_local_wall_clock_for_the_model：UTC 07:00+00:00 在 Asia/Shanghai 下渲染为 15:00

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
backend	96.57% <100.00%> (+<0.01%)	⬆️
frontend	91.05% <ø> (+0.33%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...meflow/intelligence/conversation/schedule_tools.py	100.00% <100.00%> (ø)

... and 12 files with indirect coverage changes

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[fennoai]

审阅了固定提交范围 718477427b7f494762b6f4ee9887d6ffdbdb1b82...d5a307c3ce9c5defd24b448cd18d19ff3964c148，包括组合式 schedule 工具序列化及其测试。新增的 starts_at_local 使用日程自身的 IANA 时区转换存储瞬时值，与 realtime 侧现有契约一致；未发现由本 PR 引入的、达到报告阈值的正确性或回归问题。

验证：目标文件通过 Python 编译检查和 git diff --check。无法运行 pytest/ruff，因为当前环境未安装这些命令。

[gac0812]

ok