Port the federation worker and scheduled jobs to Node.js

[dahlia]

Problem

The background process uses Deno.cron and Deno.addSignalListener to schedule maintenance jobs, drain the Fedify queue, and coordinate graceful shutdown. These runtime concerns are mixed with the job implementations.

Tasks

• Extract each scheduled job into an independently callable operation.
• Define schedule metadata separately from job logic.
• Add a Node.js scheduler adapter with abortable shutdown.
• Replace Deno signal handling with Node.js process-signal handling.
• Preserve PostgreSQL advisory locks, leasing, replica safety, and queue-drain behavior.
• Revalidate Fedify queue startup and shutdown semantics.
• Add tests for overlapping ticks, skipped locks, failures, cancellation, and graceful termination.

Acceptance criteria

• The federation queue and every scheduled job run under Node.js.
• Multiple worker replicas preserve the existing concurrency and idempotency guarantees.
• Shutdown stops new ticks, drains active work within the documented policy, and exits cleanly.
• The worker remains a separate process and is not placed behind a load balancer.

AI assistance: This issue was drafted and organized with Codex after a maintainer-led code audit.