perf: optimize EventCounterService.GetEvaluationTimeseriesCount

[nnnkkk7]

Describe the feature you'd like

Problem

API is a bit slow for multi-variation and long time-range queries.

Root Causes

1. Excessive Redis Operations: Individual Redis calls per variation (N×24 hours)
2. Inefficient DB Indexing: Missing composite index causes full table scans
3. HyperLogLog Overhead: Temporary key creation/deletion overhead

Background

No response

Alternative solutions

No response