Validation webhooks for external logic

[zeevdr]

Description

Explore support for external webhook calls during field validation. Covers cases where validation logic can't be expressed declaratively — it lives in another service (e.g., "is this SKU valid?" requires checking an inventory API, "is this region available?" requires an infrastructure service).

Key questions

• Define webhook configuration model — per-field? per-schema? per-group?
• Design request/response contract (field value, field metadata, tenant context → valid/invalid + message)
• Failure modes: what happens when webhook is unreachable? (fail-open vs fail-closed, configurable?)
• Performance: timeout limits, caching strategies, async vs sync validation
• Security: webhook authentication (HMAC, mTLS, bearer token), payload sensitivity
• Determine if this overlaps with or extends into approval/audit workflows
• SDK impact: webhooks are server-side only — how do SDKs surface webhook validation errors?
• Consider batching: if multiple fields have webhooks, call them in parallel?

Context

Related to CEL expressions (#76) and externally managed fields — these are complementary. CEL handles internal complexity without network calls; webhooks handle external validation logic that requires calling another service.

[zeevdr]

Related discovery issues: #76 (CEL expressions), #78 (externally managed fields)