feat(audit): meter audit events that go missing before the store write (NAN-6472)

[pfreixes]

• An audit event that vanishes upstream of the store is now countable. store.record() already meters every ClickHouse write attempt, so a failed write is alertable — but the two catch blocks that fire before it, when event construction or target resolution throws, were log-only. An event we intended to record disappearing was invisible to metrics.
• Dropped and degraded are separate counters, because they are different failures. The finish listener is registered before target resolution runs, so throwing before registration loses the event outright, while throwing during resolution still emits it with no target. nango.audit.emit.dropped therefore stays the single number to alert on for events actually lost, and nango.audit.resolve.failed covers the recorded-but-incomplete case.

Both carry a source tag so the dedicated auth, sync-command and MFA middlewares report distinctly once they land, and so NAN-6471's shared emit tail can absorb both increments without changing the metric contract.

Deliberately not metered

• DropAuditStore.record() returns Ok while discarding, so any deployment without CLICKHOUSE_URL drops every event silently. That is correct behaviour for self-hosting rather than a fault, and no counter here sees it.
• emit()'s early return when there is no account — unreachable in practice, since the listener is only registered when one exists.

Test plan

• ts-build, npm run lint (exit 0), prettier clean
• 10/10 middleware unit tests, 5/5 audit middleware integration tests
• One test per path: resolution failure (degraded — asserts the event is still recorded with its account and outcome), gate failure and emit failure (both dropped — assert nothing reaches the store). Each asserts the counter that should fire and the one that should not
• Verified non-vacuous: removing both increments reds exactly the three new tests and leaves the other seven green
• Verified the dropped/degraded split is load-bearing: flattening it to a single counter reds exactly the gate-failure test
• After deploy: confirm both counters report, and set a threshold on nango.audit.emit.dropped — audit volume is low enough that any non-zero value is worth looking at

🤖 Generated with Claude Code

[linear-code]

NAN-6472

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}