feat: 统计数据优化 - 支持细粒度时间范围和多维度分析

- 新增 StatsHourly/StatsDaily 预聚合表，支持任意时区的精确统计
- 实现 UTC datetime 范围查询策略，边界数据实时聚合
- 新增统计 API：用户/API Key 维度、成本分析、性能百分位、错误分类
- 新增前端页面：成本分析、性能分析、用户统计
- 新增 TimeRangePicker 组件和统计可视化组件
- 优化 Dashboard 和 Usage 页面支持时间范围筛选

Close #135

tests/services/usage/test_error_classifier.py

@@ -0,0 +1,27 @@
+import pytest
+
+from src.core.enums import ErrorCategory
+from src.services.usage.error_classifier import classify_error
+
+
+@pytest.mark.parametrize(
+    "status_code,error_message,status,expected",
+    [
+        (429, None, None, ErrorCategory.RATE_LIMIT),
+        (401, None, None, ErrorCategory.AUTH),
+        (404, None, None, ErrorCategory.NOT_FOUND),
+        (None, "maximum context length exceeded", None, ErrorCategory.CONTEXT_LENGTH),
+        (None, "content_filter triggered", None, ErrorCategory.CONTENT_FILTER),
+        (None, "rate limit reached", None, ErrorCategory.RATE_LIMIT),
+        (None, "request timeout", None, ErrorCategory.TIMEOUT),
+        (None, "connection reset by peer", None, ErrorCategory.NETWORK),
+        (500, None, None, ErrorCategory.SERVER_ERROR),
+        (400, None, None, ErrorCategory.INVALID_REQUEST),
+        (200, None, "cancelled", ErrorCategory.CANCELLED),
+        (None, None, None, ErrorCategory.UNKNOWN),
+    ],
+)
+def test_classify_error(
+    status_code: int | None, error_message: str | None, status: str | None, expected: ErrorCategory
+) -> None:
+    assert classify_error(status_code, error_message, status) == expected