Refactor: stat service deprecated stats

[RC-CHN]

Modifications / 改动点

• StatService.get_stat (the endpoint behind the dashboard stats page) still relied on three methods that were deprecated in v4.0.0: get_base_stats, get_grouped_base_stats, and get_total_message_count. Every stats page load emitted a burst of DeprecationWarnings — one per method call plus one per row from instantiating the deprecated po.Platform/Stats dataclasses — and each call spawned a throwaway thread running asyncio.run to bridge sync to async.

• This PR migrates get_stat to query PlatformStat directly while keeping the response shape byte-identical.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Refactor dashboard stat retrieval to use PlatformStat directly while preserving the existing response schema for the stats endpoint.

Enhancements:

• Simplify StatService.get_stat to compute per-platform aggregates, global totals, and hourly time-series directly from PlatformStat rows instead of deprecated helper methods.

Tests:

• Add unit tests for StatService.get_stat to verify aggregation logic, window behavior, and response structure for non-empty and empty stat windows.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="tests/unit/test_stat_service.py" line_range="19-28" />
<code_context>
+    }
+
+
+@pytest.mark.asyncio
+async def test_get_stat_empty_window(temp_db):
+    """A window with no rows yields empty platform stats but keeps the total."""
+    old_ts = datetime.now() - timedelta(hours=2)
</code_context>
<issue_to_address>
**suggestion (testing):** Add a test for a completely empty database window to cover the `total_messages == 0` path.

`test_get_stat_empty_window` only covers a window with no in-range rows while still having an out-of-window row. We’re missing coverage for the case where `PlatformStat` has no rows at all, to exercise `func.coalesce(sum(...), 0)` and verify `message_count` is `0` with an all-zero series. Please add a small test with an entirely empty DB (no inserts) to confirm this default behavior.
</issue_to_address>

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[sourcery-ai]

suggestion (testing): Add a test for a completely empty database window to cover the total_messages == 0 path.

test_get_stat_empty_window only covers a window with no in-range rows while still having an out-of-window row. We’re missing coverage for the case where PlatformStat has no rows at all, to exercise func.coalesce(sum(...), 0) and verify message_count is 0 with an all-zero series. Please add a small test with an entirely empty DB (no inserts) to confirm this default behavior.