Extract webhook activity mapper

[GarbageBoy8]

Summary

Closes #46

Extracts webhook-to-activity mapping from the webhook route handler into a dedicated activity
mapper service.

Changes

• Moved webhook event mapping logic out of apps/dashboard/app/api/webhooks/route.ts
• Added a focused mapper module for converting validated webhook payloads into ActivityEvent | null
• Kept unsupported webhook event types returning null
• Added tests covering membership, pass, guild, and verification event mappings
• Added coverage for payloads missing optional display fields such as names

Why

This keeps the route handler focused on HTTP concerns like validation, idempotency, storage,
and responses, while making webhook activity mapping easier to test, reuse, and extend safely.

Testing

• Added activity mapper tests for supported webhook event types
• Verified existing webhook response behavior remains unchanged

[Lakes41]

Good Job, Do resolve conflicts @GarbageBoy8

[Lakes41]

@GarbageBoy8 Do resolve conflicts, I might have to take you off when it 24 hours