Schedule: Fix panic on duration conversion with proper error handling

[PedramNavid]

Problem

Using .unwrap() will panic if the duration is too large to convert (> ~584 years).

Locations: src/core/schedule.rs:204 and src/core/schedule.rs:237

let next = after + chrono::Duration::from_std(*duration).unwrap();

Recommendation

Return an error instead:

let chrono_duration = chrono::Duration::from_std(*duration)
    .map_err(|_| ScheduleError::InvalidInterval("duration too large".into()))?;
let next = after + chrono_duration;

Apply this fix in both next_after() (line 204) and next_n_after() (line 237).

Priority

Critical - Panics should be avoided in library code.

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Created from PR review: #28 (comment)