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/services/import_service.py

@@ -21,8 +21,11 @@ OPTIONAL_COLUMNS = {
 }
 
 
+from app.config import settings
+
+
 class ImportService:
-    MAX_ROWS = 10000
+    MAX_ROWS = settings.MAX_EXCEL_ROWS
 
     @staticmethod
     def parse_xlsx(file_bytes: bytes) -> Tuple[List[Dict[str, Any]], List[str]]: