Columnar span store for scale (ClickHouse / Timescale)

[AKogut]

Context

Postgres JSONB is fine for MVP; span-level, high-cardinality data at scale wants a columnar store behind the same repository interface.

Scope

• Benchmark Postgres vs ClickHouse/Timescale for span queries at volume
• Implement span store behind existing repository seam
• Ingestion sharding / partitioning strategy
• Migration/backfill plan + dual-write window

Acceptance criteria

• Trend/trace queries meet latency targets at 10–100x MVP volume; core code unchanged behind the seam