feat(O9): Postgres internal observability
- DB pool size/idle/active gauges updated every 60s (background task) - DB query total/errors counters defined - DB query/transaction duration histograms defined - Table row count gauges (entity, edge, chunk) updated periodically - Deep PG stats (pg_stat_*, pg_statio_*) collected by pg_exporter - Metrics: PG1-PG33 (app-visible subset, rest from pg_exporter)
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