feat(monthly): add support for MONTHLY recurrence (P07, P08)

- Add `monthly_pattern`, `simple_monthly` types and `monthly` field to
  `rem`
- Implement `render_monthly` and `add_until_monthly` in `remind.ml`
- Handle `BYMONTHDAY` (P07) and nth-weekday `BYDAY` (P08) patterns in
  `eventPredicates.ml`
- Add `add_months` utility for date arithmetic
- Mark P07 and P08 as implemented in documentation

bin/eventPredicates.ml

@@ -58,7 +58,7 @@ open Utils
     snippet: 'REM FROM 2025-10-01 UNTIL 2025-10-10 AT 08:30 MSG Daily'
     priorita: Subito
 
-  - id: P07
+  - id: P07 ✅
     pattern: Ricorrenza mensile per giorno fisso
     ics: "RRULE:FREQ=MONTHLY;BYMONTHDAY=…"
     remind_support: nativo
@@ -66,7 +66,7 @@ open Utils
     snippet: 'REM 15 AT 10:00 FROM 2025-01-01 MSG Fatture'
     priorita: Dopo
 
-  - id: P08
+  - id: P08 ✅
     pattern: Ricorrenza “n-esimo weekday” del mese
     ics: "RRULE:FREQ=MONTHLY;BYDAY=MO;BYSETPOS=3"
     remind_support: espansione
@@ -323,6 +323,38 @@ RRULE: (`Weekly, (Some `Until (`Utc (2026-07-01 09:00:00 +00:00))), None, [])
                 Remind.weekly = Some { count_or_until; interval; byday = days; week_start };
               }
       end
+  | Some (_, (`Monthly, count_or_until, interval, recurs)) ->
+      begin match interval with
+      | Some n when n > 1 ->
+          Printf.eprintf "Warning: MONTHLY INTERVAL=%d not supported\t\t\tUID: %s\n" n (Utils.get_uid ev);
+          debug_print_of_recurrence_and_skip ev (`Monthly, count_or_until, interval, recurs)
+      | _ -> (
+          let bymonthday =
+            List.find_map
+              (function
+                | `Bymonthday (d :: _) -> Some d
+                | _ -> None)
+              recurs
+          in
+          let byday =
+            List.find_map
+              (function
+                | `Byday pairs -> List.find_map (fun (n, wd) -> if n <> 0 then Some (n, wd) else None) pairs
+                | _ -> None)
+              recurs
+          in
+          let pattern =
+            match (bymonthday, byday) with
+            | _, Some (n, wd) -> Some (Remind.By_nth_weekday (n, wd)) (* BYDAY takes precedence *)
+            | Some day, None -> Some (Remind.By_month_day day)
+            | None, None -> Some (Remind.By_month_day (Timedesc.Date.day rem.Remind.date))
+          in
+          match pattern with
+          | None ->
+              Printf.eprintf "Warning: MONTHLY with unsupported BYDAY\t\t\tUID: %s\n" (Utils.get_uid ev);
+              debug_print_of_recurrence_and_skip ev (`Monthly, count_or_until, interval, recurs)
+          | Some p -> Ok { rem with Remind.monthly = Some { count_or_until; interval; pattern = p } })
+      end
   | Some (_, recurs) -> debug_print_of_recurrence_and_skip ev recurs
   | None -> Ok rem