fix(slack): Add user_change event handler and include mapped individuals in admin

[bokelley]

Summary

• Add handler for Slack user_change events to capture profile updates in real-time (display name, real name, email, deleted status, timezone)
• Add slack_tz_offset to handleTeamJoin for consistency with sync
• Include invalidateUnifiedUsersCache in user_change handler
• Fix admin users query to include "mapped individuals" - users who have a WorkOS user ID but no organization membership

Problem

1. Missing profile updates via webhooks: We only handled team_join events, missing all subsequent profile changes. This caused the need for frequent manual syncs (1311 users updated in last sync).

2. Invisible mapped individuals: Users like Brian May who have a WorkOS user ID but no organization membership were falling through the cracks - excluded from both the AAO users list (no org membership) and the Slack-only list (has workos_user_id).

Changes

server/src/slack/events.ts

• Add SlackUserChangeEvent interface and handler
• Add slack_tz_offset to handleTeamJoin for consistency
• Invalidate unified users cache on profile changes

server/src/routes/admin/users.ts

• Include mapped individuals (have workos_user_id but no org) with mapping_status: 'mapped'
• Show their name and email from Slack data

Test plan

• Deploy and verify Brian May appears in admin search for "may"
• Add user_change event subscription in Slack app settings
• Verify profile updates trigger the new handler (check logs)

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