feat(slos): latency ladder counters on the service overview rollups - #2294

🟢 Confidence 10/10 · safe to merge. Column order, frozen DDL and generated schema all agree, and the ladder, its zero coverage on old buckets and the v50 to v51 edge are each pinned by a test.
_{no findings · tests covered · risk high}

Adds nine sampling-weighted SLO latency ladder counters to both service overview rollups, in migration 0059, the Tinybird views (deployed by ALTER) and the v51 local schema, with tests for the view math, the zero coverage of pre-0059 buckets and the chDB edge. Safe to merge.

• Migration 0059 adds the eight rungs plus EstimatedLadderSpanCount and recreates both overview views
• SLO_LATENCY_LADDER_SQL derives the rung SQL from SLO_LATENCY_THRESHOLDS_MS; both views deploy by alter
• Local store goes to v51 with addColumns and view drops in the step, plus a v51 snapshot

Before merge

• Migration · Check it is safe on production data and with the code still deployed · packages/domain/src/clickhouse/migrations/0059_service_overview_slo_latency_ladder.ts
• Warehouse · Deploy the schema before the code that reads it · packages/domain/src/tinybird/datasources.ts
• Manual · Watch the first Tinybird branch deploy: both overview views must apply by ALTER with no rebuild; the author flags this unverified

_{79897a7 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}