Alpha preview

The first release with enough scaffolding around it to run a real product. We had the composer and the scheduler working, but no eyes on the runtime -- this is where we wired up monitoring, logging, and the first real safety nets.

Added

• Scheduling dispatcher -- the cron worker that actually picks up due posts, calls the platform adapters, and records the result.
• Sentry, pino, and CI -- error monitoring, structured logs with request IDs, and a GitHub Actions pipeline that runs typecheck, lint, and tests on every PR.
• Error boundaries -- the app no longer blanks out on a render-time error; instead users see a recoverable boundary with a "try again" link.