Restore performant per-period project usage charts via ClickHouse + feature flag

Restore performant per-period project usage charts via ClickHouse + feature flag targets a $20.0B = 5M developer teams x $4,000 ACV (annual tooling/observability spend per team) total addressable market with medium saturation and a year-over-year growth rate of 18% (observability & developer tooling combined CAGR estimate).

Key trends driving demand: Consolidation of infra & product analytics -- companies want fewer vendor integrations and prefer integrated telemetry on platform homepages, increasing demand for built-in charts.; Shift to columnar OLAP stores (ClickHouse, Druid) -- dramatically lowers latency/cost for slices of high-cardinality telemetry, enabling interactive dashboards previously impractical.; Developer experience focus -- product and infra teams expect actionable, at-a-glance metrics on their project homepages to speed decision cycles and reduce context switching..