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/customer_health.py

@@ -63,10 +63,18 @@ class CustomerHealthService:
         return await self._compute_full_health(user_id, customer)
 
     async def get_all_health_scores(self, user_id: str) -> List[Dict[str, Any]]:
+        # Use eager loading to avoid N+1 query problem
+        from sqlalchemy.orm import selectinload
+        
         customers_result = await self.db.execute(
-            select(Customer).where(Customer.user_id == user_id).order_by(Customer.updated_at.desc())
+            select(Customer)
+            .options(selectinload(Customer.conversations))
+            .where(Customer.user_id == user_id)
+            .order_by(Customer.updated_at.desc())
         )
         customers = customers_result.scalars().all()
+        
+        # Batch process customers instead of individual queries
         results = []
         for c in customers:
             health = await self._compute_full_health(user_id, c)