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/core/utils.py

@@ -1,6 +1,8 @@
 """Shared utility functions"""
 import uuid
-from typing import Any
+from typing import Any, Optional
+from sqlalchemy import select, func
+from sqlalchemy.ext.asyncio import AsyncSession
 
 
 def validate_uuid(value: str) -> str:
@@ -24,4 +26,35 @@ def sanitize_for_logging(value: str) -> str:
     # Remove common sensitive patterns
     import re
     value = re.sub(r'[^a-zA-Z0-9\s\-_.,:;!?\'"]', '', value)
-    return value[:200]  # Limit length for log safety
+    return value[:200]  # Limit length for log safety
+
+
+def paginate_query(query, page: int = 1, size: int = 20) -> dict:
+    """
+    Paginate a SQLAlchemy query and return results with metadata.
+    
+    Args:
+        query: Base SQLAlchemy query
+        page: Page number (1-indexed)
+        size: Items per page
+        
+    Returns:
+        Dictionary with items, total, page, size, pages
+    """
+    from math import ceil
+    
+    if page < 1:
+        page = 1
+    if size < 1 or size > 100:
+        size = 20
+    
+    offset = (page - 1) * size
+    total_query = select(func.count()).select_from(query.subquery())
+    
+    return {
+        "items": query.offset(offset).limit(size).all(),
+        "total": total,
+        "page": page,
+        "size": size,
+        "pages": ceil(total / size) if total > 0 else 0,
+    }