* fix(server): kill the child process when a spawned duplex stream is destroyed
A failed database backup left pg_dump running. The stream was destroyed but
the child was not, so the dump sat idle in transaction holding AccessShareLock
on every table until someone terminated it by hand. One report had it survive
five days and block DDL from another application on the same Postgres.
Destroying the duplex now kills the child, which also covers pipeline()
tearing every stream down when one of them fails.
The backup service additionally holds references to the streams it spawns so
the error path can destroy them. createWriteStream can throw before pipeline()
takes ownership, which is how the reported ENOENT on an unmounted backup volume
left nothing referencing the running pg_dump.
* chore(server): drop the explanatory comments from the backup teardown fix
Requested in review. The reasoning lives in the commit message and the PR
body, which is where it belongs.
