[PLT-3079] docs: add the unified eventing guide

[TheMostlyGreat]

Why

Developers need one accurate path through triggers, schedules, retained events, and signed outgoing webhooks before eventing can ship.

What

• adds one /build/eventing guide with the producer → event → subscription → delivery model
• includes scoped Dashboard and REST walkthroughs for Gmail and schedules
• documents signature verification, stable delivery IDs, retries, recovery/replay, retention, and cleanup
• adds executable guide and Python receiver contract tests

Verification

• focused eventing tests: 21/21 pass
• full docs suite: 873/873 pass
• lint and typecheck: clean
• production build: /en/build/eventing rendered successfully
• GitHub checks, including Vale and generated LLM text: green

Merge gate

Ready for review. Merge and publication remain intentionally gated on the corresponding Engine eventing release.

Linear: https://linear.app/arcadedev/issue/PLT-3079

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Note

Low Risk
Documentation, examples, and tests only—no runtime product code paths change.

Overview
Adds a new Build → Events and webhooks page at /en/build/eventing that documents Arcade eventing end-to-end: producers (trigger instances, schedules, provider ingress) through retained project events to webhook subscriptions and signed deliveries.

The guide includes a seven-term model, deployment origins and scoped REST paths, a framework-free Python receiver (verify_request, SQLite idempotency inbox, receive with 400/403/500/204 semantics), Dashboard and REST walkthroughs for schedules, filtered Gmail triggers, and Slack/GitHub provider ingress, plus operations (rotate secret, recover, replay) and reliability boundaries (retries, stable webhook-id, retention).

Ships examples/eventing/receiver.py mirrored in the MDX snippet, Vitest contracts on guide content/navigation, and Python unittest coverage for signatures, rotation, duplicates, and transactional rollback. public/llms.txt lists the new page; .gitignore ignores Python cache for examples/tests.

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview	Aug 29, 2026 4:31pm