feat: add booking stats endpoint skeleton

[devin-ai-integration]

What does this PR do?

Adds a skeleton tRPC handler for booking statistics that workshop attendees will implement. The handler is intentionally incomplete - it returns placeholder values (zeros) that attendees will replace with actual Prisma queries.

The skeleton includes:

• Type definitions for input parameters (startDate, endDate, optional eventTypeId)
• Type definitions for the expected return shape (total, confirmed, cancelled, pending, noShow)
• Placeholder implementation returning zeros

Note: This is a workshop exercise skeleton, not production-ready code.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - workshop skeleton only.
• I confirm automated tests are in place that prove my fix is effective or that my feature works. N/A - skeleton with no implementation.

How should this be tested?

This is a workshop skeleton. To verify:

1. Check that the file exists at packages/trpc/server/routers/viewer/bookings/stats.handler.ts
2. Verify TypeScript types are correct
3. Workshop attendees will implement the actual Prisma queries

Human Review Checklist

• Verify this is intended as a workshop skeleton (returns hardcoded zeros by design)
• Confirm the void ctx and void input statements are acceptable for satisfying the linter on unused parameters
• Note: This handler is not yet wired to the tRPC router - may need separate integration

Checklist

• I have read the contributing guide
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have checked if my changes generate no new warnings

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Link to Devin run: https://partner-workshops.devinenterprise.com/sessions/1695e2e6fe0c4015858815ec80d6a82b
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[devin-ai-integration]

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