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)

backend/app/services/discovery.py

@@ -259,13 +259,14 @@ URL: {company_url}
             return json.loads(text)
         except json.JSONDecodeError:
             import re
-            brace = text.find("{")
+             brace = text.find("{")
             end = text.rfind("}")
             if brace >= 0 and end > brace:
                 try:
                     return json.loads(text[brace:end+1])
                 except json.JSONDecodeError:
-                    pass
+                    logger.debug(f"Failed to parse JSON from text: {text[:100]}")
+                    return None
         return None
 
     def _suggest_companies(self, product: str, market: str) -> list:

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),

backend/app/services/import_service.py

@@ -21,8 +21,11 @@ OPTIONAL_COLUMNS = {
 }
 
 
+from app.config import settings
+
+
 class ImportService:
-    MAX_ROWS = 10000
+    MAX_ROWS = settings.MAX_EXCEL_ROWS
 
     @staticmethod
     def parse_xlsx(file_bytes: bytes) -> Tuple[List[Dict[str, Any]], List[str]]:

backend/app/services/mcp_search_client.py

@@ -84,11 +84,13 @@ class MCPClientManager:
             try:
                 await self._session.__aexit__(None, None, None)
             except (BaseExceptionGroup, RuntimeError, Exception):
+                # Cleanup failed, ignore error
                 pass
         if self._ctx:
             try:
                 await self._ctx.__aexit__(None, None, None)
             except (BaseExceptionGroup, RuntimeError, Exception):
+                # Cleanup failed, ignore error
                 pass