fix: enable Addie meeting scheduling with working group auto-detection

[bokelley]

Summary

• Add working group context to Slack channels so Addie can auto-detect which working group a meeting should be scheduled for
• Add recurring meetings support with frequency, interval, count, and day-of-week options
• Add Zoom webhook endpoint for URL validation (required for Zoom app setup)
• Pass ThreadContext to meeting tools for working group auto-detection

Code Quality Improvements

• Use module-level WorkingGroupDatabase instance to avoid connection pool issues
• Use raw body middleware for Zoom webhook signature verification (critical for security)
• Move dynamic imports to module level for better performance
• Add input validation for recurrence parameters (freq, interval, count, byDay)

Test plan

• TypeScript compilation passes
• All existing tests pass
• Local server starts and serves homepage
• Dev login flow works
• Verify Zoom webhook URL validation works in production (validates after deploy)

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