Add per-org query log hot rows metric

[bill-ph]

Summary

• export duckgres_org_query_log_hot_rows per org from PostgreSQL catalog estimates
• collect row estimates and physical bytes in one leaf-partition query without scanning the query log
• clamp unknown negative reltuples estimates to zero and keep the metric observability-only
• preserve both last-good gauges on refresh failure and clear them on leadership loss

Tests

• just test-controlplane
• just lint
• manual PostgreSQL 16 smoke validation for absent tables, analyzed partitions, and fresh unanalyzed partitions

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	1	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +2 / -2
• Assertions: +4 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.