fix(scheduler): prevent NULL next_run_at from permanently dropping schedules

[molecule-ai]

Summary

Fixes #722 — three bugs in the scheduler that caused enabled schedules to silently disappear from the fire query (WHERE next_run_at IS NOT NULL AND next_run_at <= now()).

Bug 1 — fireSchedule() + recordSkipped() write NULL on ComputeNextRun error

Root cause: when ComputeNextRun fails (bad cron/tz), nextRunPtr stays nil → UPDATE SET next_run_at = $2 writes NULL → schedule excluded from every future tick.

Fix (scheduler.go): replace next_run_at = $2 with next_run_at = COALESCE($2, next_run_at). When $2 is NULL (error case), the existing DB value is preserved. Error is now logged explicitly.

Bug 2 — org importer silently discards ComputeNextRun error

Root cause: nextRun, _ := scheduler.ComputeNextRun(sched.CronExpr, tz, time.Now()) — error discarded, time.Time{} (zero) passed to INSERT.

Fix (handlers/org.go): surface the error, log it, and continue (skip the INSERT) — consistent with how prompt resolution errors are handled in the same loop.

Bug 3 — no startup repair for already-NULL'd schedules

Root cause: schedules broken by the pre-fix binary stay broken forever even after patching.

Fix (scheduler.go): Start() calls repairNullNextRunAt() once on boot — queries enabled = true AND next_run_at IS NULL, recomputes via ComputeNextRun, and UPDATEs each row. Logs count of repaired/skipped schedules.

Tests added (all pass)

Test	Covers
TestFireSchedule_ComputeNextRunError	Bug 1: UPDATE fires with COALESCE when ComputeNextRun fails
TestRecordSkipped_ComputeNextRunError	Bug 1: same invariant on the skipped path
TestRepairNullNextRunAt_RepairsRows	Bug 3: startup repair SELECTs NULL rows and UPDATEs each
TestRepairNullNextRunAt_DBError_NoPanic	Bug 3: DB error in SELECT is logged, no crash
TestOrgImport_ScheduleComputeError	Bug 2: confirms ComputeNextRun returns error for invalid inputs (skip path reachable)

Test output

ok  github.com/Molecule-AI/molecule-monorepo/platform/internal/scheduler  (14/14 tests pass)
ok  github.com/Molecule-AI/molecule-monorepo/platform/internal/handlers   (all pass)
ok  all packages — go test ./... -count=1

Test plan

• go build ./... — clean
• go test ./... -count=1 — all packages pass
• Manual: create a schedule with a valid cron expression, binary-patch to old code to NULL it, restart new binary → verify startup repair log line and schedule fires again
• Manual: org import with an invalid cron expression in org.yaml → verify schedule is skipped with log, other schedules still created

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