Add per-org query log hot bytes metric

[bill-ph]

Summary

• export duckgres_org_query_log_hot_bytes{org} from the leader-owned storage sampler
• measure heap, indexes, and TOAST across query-log leaf partitions; report zero before the table is initialized
• keep hot-byte observation separate from S3 billing, retain the last good value on refresh failure, and clear the gauge on leadership loss

Testing

• GOFLAGS=-run=TestStorageSampler just test-controlplane
• just test-controlplane
• just lint
• PostgreSQL 16 smoke validation for absent and partitioned query-log tables

[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 / -0
• Assertions: +10 / -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.