feat: sync working group posts to Slack channels

[bokelley]

Summary

Automatically share published working group posts to their linked Slack channels. When a post is published, a formatted message is sent to the working group's Slack channel (if configured).

• Add notifyWorkingGroupSlackChannel() for posting to specific channels using Block Kit formatting
• Add notifyPublishedPost() wrapper that checks privacy settings and channel configuration
• Update committees.ts routes to use new notification function (3 locations)
• Add notifications to content.ts for propose/approve endpoints (2 locations)
• Skip members-only posts to respect privacy

Closes #719

Test plan

• Create a working group with slack_channel_id set
• Create a published article post - verify Slack message appears in channel
• Create a published link post - verify format includes external URL
• Create a members-only post - verify NO Slack message is sent
• Create a draft, then publish it - verify Slack message appears
• Create post in group without channel configured - verify no error

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