[Feature] Optional "notify me and I'll publish manually" mode (Buffer-style)

[caomicc]

Summary

Today TryPost is auto-publish only: schedule a time, TryPost publishes via the
network's official API, and only notifies on failure. Some creators need the
opposite workflow — where a post is prepared and staged on schedule, but the
actual publishing is done by hand in the native app (e.g. Instagram), and TryPost
sends a "this post is due now — go post it" notification instead of publishing.

Why

• On Instagram, some actions are only possible from the native app and are not
  exposed via the Graph API at publish time (e.g. adding the in-app music/audio
  sticker to a carousel so it becomes eligible for the Reels tab).
• Buffer and Later both ship this pattern ("notify me" / "notification
  publishing"); Buffer users migrating to TryPost lose the capability.

Proposed behavior

• Per-post toggle: auto-publish (default) vs manual (notify-only).
• In manual mode, when the scheduled time arrives TryPost does not push to
  the network; instead it sends a notification (email/push/webhook) with the
  caption + media links so the user can post in-app.
• Post stays in a "ready" state until the user marks it published (or it can
  auto-close after N hours).

Affected area

Scheduling / publish worker + notification delivery.