GL-336: Normalize response field casing + unify error-body shape

[discodone]

Review source

Round 14 (Opus 4.8 + Gemini) — API Design.

Problem

Field-casing inconsistency: webhook responses use tenantId/workspaceId (camelCase) while API-key responses use workspace_id/created_at (snake_case). Error-body shape drifts: the 500 handler returns {error, message} while everything else returns {error, errorCode, reason}.

Fix

• Decide on ONE convention (snake_case is more Pythonic and matches most of the API) and apply it everywhere
• Migrate webhook response schemas from camelCase to snake_case (or add Pydantic alias for backward compat with a deprecation note)
• Unify ALL error responses to {error, errorCode, reason} including the 500 handler
• Update OpenAPI spec + SDK models to match

Acceptance Criteria

• All response fields use consistent casing (snake_case)
• All error responses (including 500) use {error, errorCode, reason}
• Contract test asserts error-body shape consistency
• mypy clean, ruff clean, tests >= baseline