Registers a URL that will receive HMAC-SHA256-signed POSTs on events. The returned secret is shown ONLY ONCE — store it to verify the X-Nexus-Signature signature.

Available events: analysis.completed, analysis.failed, credits.low, normativa.updated (Spain indexed at 100%, operational event; other jurisdictions still on rails), citation.broken and analysis.flagged.

• citation.broken fires when anyBroken === true, that is, when the analysis contained ≥1 verifiable broken citation (mismatch + not_found + derogated > 0). The states pending (not parseable) and not_verifiable (jurisdiction with no active corpus) do NOT count as broken and never fire the event.
• analysis.flagged (the «needs human review» umbrella) fires when anyBroken === true OR when verification could not run (verificationError, fail-safe). It carries reasons: string[] (e.g. ["broken_citations"], ["verification_error"]). The nodoBVerdict travels as informational data inside quality but is NOT by itself a trigger.

Governance events (v1.30.0): apikey.created, apikey.revoked, apikey.updated, apikey.spend_cap_changed, spend_cap.reached and byo_provider.rejected are low-frequency lifecycle events (key management, spend-cap enforcement, BYO fail-closed rejection). Every event delivered here is also durably readable via the pull API GET /api/v1/events (replay/backfill), even for accounts with no push endpoint registered.

Operational governance events (v1.31.0): provider.unreachable (a BYO provider was unreachable — payload provider_id/error_class/jurisdiction), verify.engine.degraded (the citation-verification engine degraded on a metric — payload degraded_metric/sample_size/jurisdiction) and rate_limit.sustained (sustained 429s over a window — payload window/count_429/rpm_limit/key_id). Zero-Retention: these payloads carry only those diagnostic fields — never url, key material or request body. Also durably readable via GET /api/v1/events and the signed SIEM export GET /api/v1/audit/export.

Only API-key-authenticated traffic has a webhook destination: citation.broken/analysis.flagged are emitted for jobs and synchronous API-key calls, not for web-session traffic.

Every delivery carries a stable event_id (uuid) in the body — identical across retries and shared by all subscribed endpoints — for idempotent deduplication in SIEM. It is an alias of id; the X-Nexus-Event-Id header mirrors that value (unlike X-Nexus-Delivery-Id, which is per-endpoint/per-retry and is NOT usable as a dedupe key).

Signature: X-Nexus-Signature: t=<unix-ts>,v1=<hex-hmac> where hmac = HMAC-SHA256(secret, t + '.' + rawBody). Exponential retries up to 5 times (1m, 5m, 30m, 2h, 12h).