Enable mapping slack channels to programs & post delivery notes in right channel

[mveytsman]

Summary

This PR adds per-program Slack channel configuration for delivery note notifications. Previously, all delivery notes were posted to a single hardcoded Slack channel. Now, each program can optionally specify its own Slack channel, allowing for better organization of notifications across different programs.

Changes

Database

• Added slack_channel_id field to the programs table to store the Slack channel ID for each program

Backend

• SlackApi module: Added list_channels/0 function to fetch available Slack channels from the API
• SlackApi.Http: Implemented get!/2 to support GET requests to the Slack API
• Messaging.Slack.DeliveryNotes:
  • Modified post_message!/2 to accept a channel parameter
  • Updated notify_note_created!/1 and notify_note_resolved!/2 to use program-specific channel
  • Added get_channel_id/1 helper that retrieves the channel from the program, falling back to the default config
    channel if none is set

Frontend

• Program form: Added Slack channel selector dropdown
  • Fetches and displays available Slack channels (filtered to "campaigns-*" and "api-playground" channels)
  • Allows programs to select a specific channel or leave it unset to use the default
  • Gracefully handles Slack API errors by showing an empty list

Behavior

• When a delivery note is created or resolved, the system checks if the associated program has a slack_channel_id
  configured
• If configured, the notification is posted to that program's channel
• If not configured, the notification falls back to the default channel from config
• Programs can be updated to use different channels at any time through the program edit form

[teesloane]

I'm not sure why CI is failing on a mix format - I tried formatting locally and no changes resulted.

LGTM