feat: resolve Slack usernames in Addie admin thread view

[bokelley]

Summary

• Add resolveSlackUserDisplayName and resolveSlackUserDisplayNames shared utilities to slack/client.ts
• Resolve Slack user IDs to display names in thread titles and message content mentions
• Persist resolved users to database for future lookups (avoids repeated API calls)
• Implement concurrency limiting (5 parallel requests) to avoid Slack rate limits

Changes

• server/src/slack/client.ts: New shared utilities for username resolution with DB caching
• server/src/addie/member-context.ts: Use shared utility instead of duplicated code
• server/src/routes/addie-admin.ts: Resolve all user mentions when fetching thread details
• server/public/admin-addie.html: Replace <@U...> mentions with @DisplayName in rendered messages

Test plan

• TypeScript type checking passes
• All tests pass
• Build succeeds
• Manual testing: View an Addie thread with user mentions - should show display names instead of raw IDs

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