feat(self-host): full Docker Compose self-hosting via a Miniflare runtime

[tlvenn]

Follow-up to #365, delivering the full self-hosted path discussed in #352 — run Maple outside Cloudflare, no Tinybird.

apps/api, apps/alerting, and apps/electric-sync are Workers with no plain-HTTP entry. deploy/workerd/ runs their Wrangler bundles under Miniflare in one container — deriving every binding and cron from each worker's own wrangler.jsonc at boot, so it tracks config changes here with no manual mirroring. It reaches Postgres via the Hyperdrive binding and runs the drizzle migrations (incl. the Electric publication) on startup.

docker-compose.selfhost.yml wires the whole stack — postgres + electric, clickhouse + ch-migrate + collector, workerd, the web SPA, and a Caddy proxy giving one origin (/api→api, /sync→electric-sync, /*→SPA). docs/self-hosting.md is the guide; .env.selfhost.example the config. Also removes the dead apps/api/Dockerfile (its bun run start CMD has no matching script; the api is a Worker) and reverts docker-compose.yml to the postgres+electric dev-DB role its header documents.

Validated end-to-end from a clean docker compose -f docker-compose.selfhost.yml up: ClickHouse + Postgres migrations, self-hosted login, and the Electric dashboards shape all succeed through the proxy; a smoke trace reaches the collector.

Runtime note: Miniflare is the working path today; celld is the natural target once it can run the api (TCP/Hyperdrive→Postgres, cron/scheduled, KV/Queues) — happy to help move there when those land.

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Structural choices I'd welcome your steer on:

• New docker-compose.selfhost.yml rather than reworking the root docker-compose.yml (kept as your wrangler dev dev-DB), so the two don't conflate. Happy to merge them if you'd prefer one.
• Bundled Caddy proxy for a one-origin http://localhost:3471 experience; drop it if you'd rather document a bring-your-own ingress.

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