Bug: Transaction vibration notifications not being sent for high-value transactions

Summary

Active subscribers with device tokens registered are not receiving vibration notifications for transactions above their threshold amount, even though:

• Webhooks are being received and processed ✅
• Transactions are being stored ✅
• Balances are being updated ✅
• Device tokens are registered and active ✅

Evidence

User: mcguirecaroline15@gmail.com (active subscriber)

Settings:

• vibrations_enabled: true
• threshold_amount: $50
• device_token: registered Dec 18, active
• time_zone: Europe/Madrid

Transactions that should have triggered notifications:
| Transaction | Amount | Created | vibration_sent_at |
|-------------|--------|---------|-------------------|
| Airbnb | $574.79 | Dec 18 20:34 | NULL ❌ |
| Mecalito | $76.43 | Dec 18 20:34 | NULL ❌ |
| Mercadona | $85.76 | Dec 17 20:32 | NULL ❌ |

Production logs at 20:34-20:35 show:

• Webhook received and stored ✅
• Balance snapshots updated ✅
• Widget refresh sent ✅
• NO "VIBRATION CHECK" logs ❌

Root Cause Hypotheses

1. Transactions detected as duplicates - Vibrations only process for NEW transactions. If transactions already existed in DB, they wouldn't be checked for vibrations.

1. Debug logs not captured - Vibration processing logs are at debug level which may not show in production.

1. Bug in vibration flow - processVibrationsForNewTransactions() may not be called or newTransactions array is empty.

Additional Issues Found

Broken log statements in NotificationService.js:

// Line 65 - no interpolation
logger.info('VIBRATION CHECK []: amount=$, prefs=)}');

// Line 76 - no interpolation  
logger.info('VIBRATION []: SKIP - Amount $ < threshold $');

These should use template literals with actual values for debugging.

Files to Investigate

• backend/services/NotificationService.js - Fix broken logs, check threshold logic
• backend/services/WebhookProcessorService.js - Check processVibrationsForNewTransactions() is called
• backend/services/TransactionService.js - Check how newTransactions vs duplicates are determined

Steps to Reproduce

1. Have an active subscriber with device token and vibrations enabled
2. Trigger a webhook with transactions above threshold
3. Check if vibration_sent_at gets populated
4. Check production logs for "VIBRATION CHECK" entries

Expected Behavior

Transactions above threshold should:

1. Trigger "VIBRATION CHECK" log
2. Pass threshold check
3. Send push notification
4. Update vibration_sent_at column