fix: increase API token entropy from 128 to 256 bits

Change token random bytes from 16 to 32, producing tokens with
upaas_ prefix + 64 hex characters instead of 32.

internal/models/api_token.go

@@ -15,7 +15,7 @@ import (
 )
 
 // tokenRandomBytes is the number of random bytes for token generation.
-const tokenRandomBytes = 16
+const tokenRandomBytes = 32
 
 // tokenPrefix is prepended to generated API tokens.
 const tokenPrefix = "upaas_"