feat: configurable stop grace period for applications

Adds stop_grace_period to application settings (seconds, 1-3600, default 30).
Used in place of the hardcoded docker stop -t 30 in the four places that
stop application containers: rolling update shutdown, manual stop, stop on
another server, and preview deployment stop.

Non-positive values fall back to the default via ($val > 0) ? $val : default,
with tests covering 0 and -10 so the cast does not blow up if a bad value
ever lands in the db.

Picks up Jack Coy's work from #7125 which went dormant. His commits are
squashed here with credit below.

Co-authored-by: Jack Coy <jackman3000@gmail.com>