Supabase Edge Function Debugging & Security Review

Please conduct a thorough technical diagnosis and security review of this Supabase Edge Function.

### Function Details
- **Function Name**: process-payment-webhook
- **Authorization Model**: Authenticated User (JWT / Bearer Token via RLS)
- **Observed Error / Problem**:
```text
Failed to send request: TypeError: Cannot read properties of undefined (reading "id") at server.ts:24. Browser console also reports: Access to fetch blocked by CORS policy: Response to preflight request doesn't pass access control check.
```

### Function Source Code
```typescript
import { serve } from "https://deno.land/std@0.168.0/http/server.ts"
import { createClient } from "https://esm.sh/@supabase/supabase-js@2"

serve(async (req) => {
  const { userId, planId } = await req.json()
  const supabase = createClient(
    Deno.env.get('SUPABASE_URL') ?? '',
    Deno.env.get('SUPABASE_SERVICE_ROLE_KEY') ?? ''
  )
  const { data, error } = await supabase.from('profiles').select('*').eq('id', userId).single()
  return new Response(JSON.stringify(data), { headers: { "Content-Type": "application/json" } })
})
```

### Required Review Deliverables
1. **Root-Cause Analysis**: Explain precisely why the error occurred in the Deno runtime environment.
2. **CORS & Preflight Verification**: Verify whether `OPTIONS` preflight headers and standard response headers adhere to browser security specifications.
3. **Authentication & Authorization Check**:
   - Does the function properly verify the user's `Authorization: Bearer <token>` header?
   - Is the service role key (`SUPABASE_SERVICE_ROLE_KEY`) exposed to the client or used unnecessarily where RLS would suffice?
4. **Refactored Code**: Provide the clean, idiomatic TypeScript source code with explicit error handling, typed response wrappers, and graceful fallbacks.
5. **Local Testing Command**: Provide the exact `supabase functions serve` CLI invocation to test this locally with dummy payload and mock headers.