Inngest foundation: self-hosted orchestration engine

[NiclasDev63]

What to build

Inngest becomes the org-wide background-work engine, running end to end in all three environments: the local dev server for development, a self-hosted inngest/inngest container beside the collab server (config and run history persisted to the existing Postgres) for the Docker deployment, and the app serving its Inngest functions over one API route. A trivial demo function (e.g. a heartbeat) is visible running in the Inngest dashboard in each environment.

Write ADR 0004 documenting the decision: Inngest replaces the hand-rolled lease-and-chain cron pattern for background work, self-hosted (not SaaS, not Vercel Queues/Workflows — record the rejected alternatives and why), with the event key / signing key contract between app and server.

Acceptance criteria

• pnpm dev workflow includes a documented way to run the Inngest dev server locally, and the demo function executes through it
• docker-compose includes the Inngest server container backed by the existing Postgres, and the app container connects to it via env vars
• The app exposes the Inngest serve route and registers at least one working function with retries observable in the dashboard
• ADR 0004 is committed and covers the alternatives considered and the self-hosting decision
• Environment variable requirements (event key, signing key, base URL) are documented alongside the other required env vars

Blocked by

None — can start immediately.

Target branch

Base all PRs for this ticket on the epic branch knowledge-sync, not main. The epic branch integrates every Knowledge Sync ticket and merges to main as one reviewed unit before production deploy.

[NiclasDev63]

Implemented in #18, based on the knowledge-sync epic branch as the ticket specified.

All five acceptance criteria are met:

• pnpm dev:inngest runs the Inngest dev server locally, documented in docs/setup.md with a troubleshooting entry; heartbeat executes through it
• docker-compose.yml runs inngest/inngest:v1.44.0 on its own inngest database on the existing Postgres, with the app connecting via env vars
• /api/inngest serves the functions; heartbeat has retries: 2, and sending scibly/heartbeat.requested with { "fail": true } shows all three attempts in the dashboard
• ADR 0004 records the decision, with Inngest Cloud and Vercel Queues/Workflows as the rejected alternatives and why
• INNGEST_BASE_URL, INNGEST_EVENT_KEY, and INNGEST_SIGNING_KEY are documented in both .env.example files, docs/setup.md, and docs/docker.md

Closing now as asked. Note the PR targets knowledge-sync rather than the default branch, so the Closes #8 keyword would not have auto-closed this on merge.