feat: Use `Duration.shift()` instead of adding durations when resolving datetimes

[BelhsanHmida]

Problem

In the schema, datetimes are currently derived by adding durations:

predict_start = start_date + train_period

This assumes the duration behaves like a fixed-length timedelta. However,
DurationField may return an isodate.duration.Duration (e.g. P1M, P1Y),
for which direct addition is not supported and calendar semantics are required.

Proposed change

When resolving datetimes from ISO-8601 durations, use the duration’s
calendar-aware shift method instead of direct addition.

from isodate.duration import Duration

if isinstance(train_period, Duration):
    predict_start = train_period.shift(start_date)
else:
    predict_start = start_date + train_period