make bucket-scan wordlist opt-in (default off)

Discovery is now driven purely by incoming CT domains (stem + suffix
expansion); the hardcoded generic-name wordlist is only used when
BUCKET_SCAN_WORDLIST=true.

.env.example

@@ -35,3 +35,4 @@ BUCKET_SCAN_QPS=50
 BUCKET_SCAN_WORKERS=16
 BUCKET_SCAN_QUEUE=65536
 BUCKET_SCAN_DO=false
+BUCKET_SCAN_WORDLIST=false