Added richer time syntax support:
- Named weekdays: every wednesday at 00:00, every monday,friday at 18:30

- Lists/ranges: every day of week in 1,3,5 at 12:00, every day of month in 1..5 at 09:00

- Multiple daily times: every day at 08:00,12:00,18:00

- Time windows: every 15 minutes from 09:00 to 17:00

- Nth/last weekday of month: every 2nd monday of month at 10:00, every last friday of month at 22:00

- Relative keywords: every weekday at 09:00, every weekend at 11:00, every month on last-day at 23:55

- Date constraints: between YYYY-MM-DD and YYYY-MM-DD

- Timezone support per job: timezone Europe/Berlin

- One-shot jobs: at 2026-06-10 14:30

- Optional jitter: jitter 30s / jitter 2m

- Optional cron backend: cron: 0 0 * * 3

New validation command:
- /srvcron validate <time-expression> shows parsed interpretation + next runs.

Improved schedule preview:
- /srvcron checkschedule <job> [count] to view upcoming executions.