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/api/v1/auth.py

@@ -155,7 +155,6 @@ async def login(
 async def guest_login(request: Request, db: AsyncSession = Depends(get_db)):
     # Rate limiting: max 5 guest logins per IP per 15 minutes
     from app.core.redis import get_redis
-    import time
     
     client_ip = request.client.host if request.client else "unknown"
     cache_key = f"guest_login:{client_ip}"
@@ -163,8 +162,8 @@ async def guest_login(request: Request, db: AsyncSession = Depends(get_db)):
     try:
         redis_client = await get_redis()
         now = int(time.time())
-        window = 900  # 15 minutes
-        limit = 5
+        window = settings.GUEST_LOGIN_WINDOW  # 15 minutes
+        limit = settings.GUEST_LOGIN_LIMIT
         
         # Get count of logins in current window
         count = await redis_client.get(cache_key)
@@ -180,6 +179,8 @@ async def guest_login(request: Request, db: AsyncSession = Depends(get_db)):
         pipe.expire(cache_key, window)
         await pipe.execute()
         
+    except HTTPException:
+        raise
     except Exception:
         # If Redis is down, proceed without rate limiting
         pass

backend/app/api/v1/customer.py

@@ -136,7 +136,10 @@ async def delete_customer(
     return {"message": "Customer deleted"}
 
 
-MAX_UPLOAD_SIZE = 10 * 1024 * 1024  # 10MB
+from app.config import settings
+
+
+MAX_UPLOAD_SIZE = settings.MAX_UPLOAD_SIZE
 
 @router.post("/import")
 async def import_customers(

backend/app/api/v1/product.py

@@ -102,7 +102,10 @@ async def import_products(
 ):
     from app.services.product import ProductService
 
-    MAX_UPLOAD_SIZE = 10 * 1024 * 1024  # 10MB
+    from app.config import settings
+
+
+MAX_UPLOAD_SIZE = settings.MAX_UPLOAD_SIZE
 
     filename = file.filename or "unknown"
     file_size = 0