Add MixpanelService with hashed phoneNumber in spamBlocked() (FRE-4519)

Create MixpanelService that uses FieldEncryptionService.hashPhoneNumber()
to SHA-256 hash phone numbers before sending to Mixpanel analytics.

- Implement spamBlocked() method with phone number hashing
- Add 16 unit tests verifying hash correctness and API behavior
- Export service from package index

Co-Authored-By: Paperclip <noreply@paperclip.ing>

services/spamshield/src/index.ts

@@ -1,4 +1,5 @@
 export * from './services/spamshield.service';
+export * from './services/mixpanel.service';
 export * from './circuit-breaker';
 export * from './config/spamshield.config';
 export * from './utils/phone-validation';