fix: additional code quality and performance improvements

Code quality:
- Remove empty except blocks with proper logging
- Create shared pagination utility function
- Remove duplicate UUID validation code
- Fix dead code in translation.py

Performance:
- Fix N+1 query in followup engine (use join instead of loop)
- Add eager loading for customer health scores
- Create database indexes for common query patterns:
  - customers: (user_id, status), (user_id, last_contact_at)
  - payment_transactions: (user_id, created_at)
  - followup_logs: (user_id, customer_id)
  - notifications: (user_id, is_read)

Configuration:
- Centralize magic numbers in config.py:
  - Payment prices
  - File upload limits
  - Rate limiting settings
  - Pagination defaults
- Update auth.py to use centralized rate limiting config
- Update customer/product imports to use centralized upload limits
- Update import_service.py to use centralized MAX_ROWS

backend/app/api/v1/product.py

@@ -102,7 +102,10 @@ async def import_products(
 ):
     from app.services.product import ProductService
 
-    MAX_UPLOAD_SIZE = 10 * 1024 * 1024  # 10MB
+    from app.config import settings
+
+
+MAX_UPLOAD_SIZE = settings.MAX_UPLOAD_SIZE
 
     filename = file.filename or "unknown"
     file_size = 0