refactor: simplify Langfuse integration with AI SDK 6 (#375)

- Remove manual token attribute setting (AI SDK 6 telemetry auto-reports)
- Use totalTokens directly instead of inputTokens + outputTokens calculation
- Fix sessionId bug in log-save/log-feedback (prevents wrong trace attachment)
- Hash IP addresses for privacy instead of storing raw IPs
- Fix isLangfuseEnabled() to check both keys for consistency

app/api/log-save/route.ts

@@ -27,6 +27,11 @@ export async function POST(req: Request) {
 
     const { filename, format, sessionId } = data
 
+    // Skip logging if no sessionId - prevents attaching to wrong user's trace
+    if (!sessionId) {
+        return Response.json({ success: true, logged: false })
+    }
+
     try {
         const timestamp = new Date().toISOString()