[Change Requests] - Send a Slack message when a WP is activated with a far-off start date

Description

Context
When a work package is activated with a start date that's noticeably different from when it was scheduled to start, the team should hear about it in Slack. We want a message in the team's channel, pinging the work package's lead and manager, that says the start date moved from the old date to the new one, with a button linking to the work package. A separate piece of work computes whether the date is "far off"; this ticket is about building and sending the message. activationBufferDays

Task
Add a new Slack notification function and call it during activation so that, when the activation's start date is flagged as outside the allowed window, the team channel gets a "start date changed" message.

Acceptance Criteria

• A new function sendActivationStartDateChangedNotification exists in backend slack.utils.ts.
• It posts to each of the work package's project team channels (team.slackId), pings the WP's lead and manager, and states the old start date → the new start date.
• The message includes a button/link to the work package page (https://finishlinebyner.com/projects/).
• It's called inside createActivationChangeRequest only when the activation is flagged as outside the buffer window
  If a work package has no lead or no manager set, the message still sends and just omits the missing ping (it doesn't crash).

Start in backend slack.utils.ts and locate createActivationChangeRequest, then trace the activation path that determines whether the start date is outside the buffer window. Add the notification function and invoke it only for flagged activations; done means each project team channel receives the date-change message with available lead or manager pings and a work package link, without failing when either person is missing.