perf: 全栈查询优化、前端缓存去重与页面可见性优化

后端:
- SQL count 查询统一改用 func.count() 子查询替代 query.count()
- Dashboard/Audit 等页面多次独立查询合并为单次聚合查询
- Provider summary 列表改为批量查询消除 N+1 问题
- DailyStats 逐天循环查询改为 CASE 分桶单次查询
- 使用 load_only() 减少不必要的列加载
- cache_decorator 支持嵌套属性路径解析(dotted vary_by)
- 多个管理/公共端点新增 @cache_result 缓存装饰

前端:
- cache.ts 新增 in-flight 请求复用、dedupedRequest、buildCacheKey
- 大量 API 调用添加前端缓存或去重
- 多个页面定时器在标签页隐藏时暂停、可见时恢复
- Auth 检查从 setInterval 改为 storage + visibilitychange 事件驱动
- 请求竞态防护(requestId 模式)

数据库:
- Usage 表新增 idx_usage_status_user_created 复合索引

alembic/versions/20260303_1730_5f1d2e3c4b5a_add_idx_usage_status_user_created.py

@@ -0,0 +1,45 @@
+"""add_idx_usage_status_user_created
+
+Add composite index on usage(status, user_id, created_at) to speed up
+interval timeline and active usage analytics queries.
+
+Revision ID: 5f1d2e3c4b5a
+Revises: 0ba031f328de
+Create Date: 2026-03-03 17:30:00.000000+00:00
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "5f1d2e3c4b5a"
+down_revision = "0ba031f328de"
+branch_labels = None
+depends_on = None
+
+INDEX_NAME = "idx_usage_status_user_created"
+TABLE = "usage"
+COLUMNS = ["status", "user_id", "created_at"]
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    result = bind.execute(
+        sa.text("SELECT 1 FROM pg_indexes WHERE indexname = :name"),
+        {"name": INDEX_NAME},
+    ).fetchone()
+    if result:
+        return
+    op.create_index(INDEX_NAME, TABLE, COLUMNS)
+
+
+def downgrade() -> None:
+    bind = op.get_bind()
+    result = bind.execute(
+        sa.text("SELECT 1 FROM pg_indexes WHERE indexname = :name"),
+        {"name": INDEX_NAME},
+    ).fetchone()
+    if not result:
+        return
+    op.drop_index(INDEX_NAME, table_name=TABLE)