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/config.py

@@ -84,5 +84,23 @@ class Settings(BaseSettings):
     PRO_MAX_PRODUCTS: int = 20
     PRO_DAILY_QUOTATIONS: int = 30
 
+    # Payment prices
+    PRO_MONTHLY_PRICE: int = 99
+    PRO_YEARLY_PRICE: int = 999
+    ENTERPRISE_MONTHLY_PRICE: int = 399
+    ENTERPRISE_YEARLY_PRICE: int = 3999
+
+    # File upload limits
+    MAX_UPLOAD_SIZE: int = 10 * 1024 * 1024  # 10MB
+    MAX_EXCEL_ROWS: int = 10000
+    
+    # Rate limiting
+    GUEST_LOGIN_LIMIT: int = 5
+    GUEST_LOGIN_WINDOW: int = 900  # 15 minutes
+    
+    # Pagination defaults
+    DEFAULT_PAGE_SIZE: int = 20
+    MAX_PAGE_SIZE: int = 100
+
 
 settings = Settings()