feat: add source file tracking to rem type

Add a `source` field to `Remind.rem` to track the originating iCalendar
filename (basename without extension). Thread the basename through
`remind_of_event` so each reminder records which file it came from.

bin/main.ml

@@ -20,6 +20,7 @@ let ical2rem ical_files =
         try
           Printf.eprintf "\nProcessing file: %s\n" filename;
           let file_content = read_file filename in
+          let basename = Filename.remove_extension (Filename.basename filename) in
           match Icalendar.parse file_content with
           | Error e ->
               if e <> ": not enough input" then prerr_endline ("Error parsing iCalendar file: " ^ e);
@@ -39,7 +40,7 @@ let ical2rem ical_files =
 
               let good_rems =
                 Map.fold ~init:[] events_map ~f:(fun ~key:uid ~data:events good_rems ->
-                    let rem_or_error = EventPredicates.remind_of_event events in
+                    let rem_or_error = EventPredicates.remind_of_event basename events in
                     match rem_or_error with
                     | Ok rem -> rem :: good_rems
                     | Error (EventPredicates.Invalid_date s) ->