perf(usage): 优化 admin usage records 查询性能

- count 查询按需 JOIN，避免不必要的表关联
- 前端 onMounted 将 stats/heatmap/records/users 全部并行加载
- 调整缓存 TTL（聚合 30s->60s，列表 10s->15s）
- 为 request_candidates 添加复合索引优化 fallback/retry 查询

alembic/versions/20260228_1448_00b9161b8729_add_request_candidates_composite_indexes.py

@@ -0,0 +1,49 @@
+"""add_request_candidates_composite_indexes
+
+Revision ID: 00b9161b8729
+Revises: 48afe197cc15
+Create Date: 2026-02-28 14:48:00.000000+00:00
+
+"""
+
+from sqlalchemy import inspect
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "00b9161b8729"
+down_revision = "48afe197cc15"
+branch_labels = None
+depends_on = None
+
+
+def _index_exists(index_name: str) -> bool:
+    bind = op.get_bind()
+    insp = inspect(bind)
+    indexes = insp.get_indexes("request_candidates")
+    return any(idx["name"] == index_name for idx in indexes)
+
+
+def upgrade() -> None:
+    # (request_id, status) - fallback/retry 查询优化
+    if not _index_exists("idx_rc_request_id_status"):
+        op.create_index(
+            "idx_rc_request_id_status",
+            "request_candidates",
+            ["request_id", "status"],
+        )
+
+    # (provider_id, status, created_at) - provider 聚合统计优化
+    if not _index_exists("idx_rc_provider_status_created"):
+        op.create_index(
+            "idx_rc_provider_status_created",
+            "request_candidates",
+            ["provider_id", "status", "created_at"],
+        )
+
+
+def downgrade() -> None:
+    if _index_exists("idx_rc_provider_status_created"):
+        op.drop_index("idx_rc_provider_status_created", table_name="request_candidates")
+    if _index_exists("idx_rc_request_id_status"):
+        op.drop_index("idx_rc_request_id_status", table_name="request_candidates")