Add placeholders for numeric month output in invoice text

Current behavior Invoice Ninja supports date placeholders such as :MONTH, :YEAR, and :MONTHYEAR in invoice text. However, :MONTH returns the month name rather than its numeric value. Proposed functionality Add two placeholders to output the month as a number, optionally with a leading zero: | Placeholder | January | October | |---|---|---| | :MONTH_NUMBER | 1 | 10 | | :MONTH_NUMBER_PADDED | 01 | 10 | These names are suggestions. Alternatives that fit the existing naming conventions would be welcome. Use case For recurring invoices, this would allow descriptions such as: Service period: :MONTH_NUMBER_PADDED/:YEAR Expected output for January 2027: Service period: 01/2027 This would allow numeric billing-period descriptions to be generated automatically without manually editing each invoice. Expected behavior • Use the same reference date as the existing :MONTH placeholder. • Support the fields where :MONTH is currently supported. &…