perf: add missing indexes for session filtering and sorting

- Add compound index on (companyId, language) for language filtering
- Add compound index on (companyId, messagesSent) for message count sorting
- Add compound index on (companyId, avgResponseTime) for response time sorting

These indexes optimize the session dashboard queries that filter by language
or sort by messagesSent/avgResponseTime, preventing full table scans.

prisma/schema.prisma

@@ -147,6 +147,9 @@ model Session {
   @@index([companyId, category])
   @@index([companyId, escalated])
   @@index([companyId, forwardedHr])
+  @@index([companyId, language])
+  @@index([companyId, messagesSent])
+  @@index([companyId, avgResponseTime])
 }
 
 /// *