handleCreatePasteMultipart silently dropped non-numeric
burn_after_reads values (err == nil guard), so burn_after_reads=abc
created a 201 paste with NO burn budget. Now returns 400 with the
error+code shape ({'code':'burn_invalid'}) matching the JSON path,
where an invalid value fails unmarshal into the int field. An empty
value stays 'unset' like the JSON path's omitempty. Verified live:
burn_after_reads=abc -> 400 burn_invalid; =2 -> 201.