Add DB migration (hour, minute, day_of_week, timezone columns) with
idempotent ALTER TABLE upgrade path for existing databases.
Extend getCronExpression with optional dayOfWeek parameter; weekly
defaults to Monday (1) when not specified.
Worker createJob now passes stored schedule fields to cron expression
and uses the subscription's IANA timezone instead of hardcoded 'UTC'.
PUT /subscription/email accepts optional hour (0-23), minute (0-59),
dayOfWeek (1-7, only for weekly), timezone (IANA). GET response
includes the new fields. Omitted fields fall back to defaults (17:00
UTC).
Closes mailship-200
