T2.2: Migrate scheduler to use domain.Schedule

Parse each job's schedule once on load (resetNextRuns) and on edit
(RefreshSchedule) via the new parseJobSchedule helper, caching the
result in a map[int]domain.Schedule keyed by job ID. prepareNextRun
now looks up the cached Schedule instead of re-parsing the string on
every call. Remove the nextRunTime wrapper that did the per-call
parsing. Drop the three scheduler_test.go tests that duplicated
coverage already in domain/schedule_test.go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/REFACTORING.md

@@ -255,7 +255,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 
 ### Phase 2 — Domain cleanup
 - [x] T2.1 — Add `src/domain/schedule.go`; Schedule value object
-- [ ] T2.2 — Migrate `scheduler` to use Schedule
+- [x] T2.2 — Migrate `scheduler` to use Schedule
 - [ ] T2.3 — Split `domain.Job` (durable) from `domain.JobRuntime` (transient)
 - [ ] T2.4 — Update `storage`: load/save Job only; move runtime init