fix(bazarr): store data in /var/lib/bazarr instead of inside /opt/bazarr (#16098)

* fix(bazarr): store data in /var/lib/bazarr instead of inside /opt/bazarr

The installer creates /var/lib/bazarr and update_script() uses it as the
"is Bazarr installed" guard, but nothing pointed Bazarr at it. Bazarr
defaults its data directory to <install dir>/data, resolved from its own
source file rather than WorkingDirectory, so a stock container kept
config/ db/ backup/ cache/ log/ restore/ in /opt/bazarr/data - the
directory fetch_and_deploy_gh_release redeploys over - while
/var/lib/bazarr stayed empty.

Pass -c /var/lib/bazarr in the unit, matching the -data= flag the other
*arr scripts use. bazarr.py re-execs its child with sys.argv[1:], so the
flag survives the restart Bazarr performs while loading its config.

update_script() gains a one-time migration for existing installs. It runs
outside the release check so containers already on the latest version are
migrated too, is skipped when the unit already names a data directory,
refuses (before stopping the service) when both locations hold data, and
copies before removing so a failed migration leaves the original database
in place.

Fixes #16097

* fix(bazarr): drop explanatory comments per review

install/bazarr-install.sh

@@ -36,7 +36,7 @@ UMask=0002
 Restart=on-failure
 RestartSec=5
 Type=simple
-ExecStart=/opt/bazarr/venv/bin/python3 /opt/bazarr/bazarr.py
+ExecStart=/opt/bazarr/venv/bin/python3 /opt/bazarr/bazarr.py -c /var/lib/bazarr
 KillSignal=SIGINT
 TimeoutStopSec=20
 SyslogIdentifier=bazarr