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/discovery.py

@@ -259,13 +259,14 @@ URL: {company_url}
             return json.loads(text)
         except json.JSONDecodeError:
             import re
-            brace = text.find("{")
+             brace = text.find("{")
             end = text.rfind("}")
             if brace >= 0 and end > brace:
                 try:
                     return json.loads(text[brace:end+1])
                 except json.JSONDecodeError:
-                    pass
+                    logger.debug(f"Failed to parse JSON from text: {text[:100]}")
+                    return None
         return None
 
     def _suggest_companies(self, product: str, market: str) -> list: