TickTick exports could contain non-numeric values in columns Vikunja
parses as integers (Priority, taskId, parentId). gocsv's strconv.ParseInt
then failed, aborting the entire import and surfacing as an internal
server error reported to Sentry (e.g. parsing "p1": invalid syntax).
Numeric ID columns now fall back to 0 for unparseable values instead of
failing the import. The Priority column, which was previously parsed but
never carried over to the imported task, is now mapped onto the task and
accepts both the plain numeric form (0, 1, 3, 5) and the "pN" form
(p1, p2, p3).
Closes#2822
ebb89ba4f3