Explorer redesign D1: rollup tables and in-process rollup worker

Spec §4.2 and §4.4.

• Flyway migration for the explorer_rollup_* tables, explorer_snapshot_wealth_day, explorer_rollup_run and explorer_rollup_coverage.
• In-process worker in lib/rollup/, run by one pod at a time through a Redis leader lease. Recomputes today and yesterday every minute and the last 7 days nightly.
• Resumable, newest-first backfill.
• Its own small DB pool.
• Read helpers in lib/sql/rollup.ts and lib/services/rollup.ts, with coverage checks.
• Category taxonomy in lib/categories.ts.
• Rollup health in /healthz.

No new deployables, and no triggers on the ledger write path.