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/followup_engine.py

@@ -287,11 +287,20 @@ class FollowupEngine:
         total = len(count_result.scalars().all())
 
         items = []
-        for log in logs:
-            customer_result = await self.db.execute(
-                select(Customer).where(Customer.id == log.customer_id)
-            )
-            customer = customer_result.scalar_one_or_none()
+        # Use join to avoid N+1 query problem
+        query = select(FollowupLog, Customer).join(
+            Customer, FollowupLog.customer_id == Customer.id, isouter=True
+        ).where(
+            FollowupLog.user_id == user_id
+        ).order_by(
+            FollowupLog.created_at.desc()
+        ).offset((page - 1) * size).limit(size)
+        
+        result = await self.db.execute(query)
+        rows = result.all()
+        
+        items = []
+        for log, customer in rows:
             items.append({
                 "id": str(log.id),
                 "customer_id": str(log.customer_id),