perf: hourly usage aggregate (usage_hourly) to remove RDS IOPS bursts from analytics

[henrypark133]

Problem

Prod RDS hits its storage ceiling (gp3 12k IOPS / 500 MiB/s) in scheduled bursts at :17–:24 each hour (nearai-stats sync) and :01 at 00/06/12/18 UTC. ~97% of burst load is admin analytics and customer usage-summary queries scanning raw organization_usage_log (43.6M rows, 12 GB heap, 25 GB indexes); month-wide windows fall into full sequential scans (~100–190 s per 30-day model-revenue call).

Plan

One derived table, usage_hourly (inference usage per UTC hour × org × workspace × key × model × provider), recomputed hourly from raw by a background job (replace semantics, data-derived progress, fast catch-up after deploy, per-day parity logs). Analytics readers switch to it; their raw aggregation SQL is deleted.

Shipping as one release, three stacked PRs:

• PR B: table + hourly recompute job + parity log (no reader changes)
• PR C: structural split of crates/database/src/repositories/analytics.rs by report family (no behavior change)
• PR D: switch admin analytics + customer usage summaries + API-key list to usage_hourly; 504 on statement timeout; lag metric

Stays on raw by design: credit_type-filtered reports, row-level history/exports, /v1/billing/costs, per-minute revenue density, per-key admission.

Context

• Spend counters (feat: maintain transactional inference and service spend counters #1116–fix: squash spend counter migrations into one idempotent, lock-guarded migration #1127) were reverted in revert: remove spend counters before they reach any database #1132 before reaching any database.
• Latency SLA metrics are owned by Per-client TTFT metrics for SLA reporting #1122.
• Documented behavior changes after PR D: ranges normalized to whole UTC hours; figures lag up to ~65 min; multi-row latency percentiles are approximate.

Success criteria

• PI read IOPS/throughput in the burst windows < 50% of the ceiling
• Admin analytics endpoints p95 < 2 s
• nearai-stats monthly streams succeed with their default deadline