(bugfix) preserve saved global backup root in Django setup form

Fix the global config edit view so default initial values are only used
when creating a new config, preventing saved backup_root values from
being hidden by form defaults.

Keep pobsync_home as an internal runtime setting instead of exposing it
in the normal Django setup form.

Mount a host backup directory into Docker at /backups and document
POBSYNC_BACKUP_ROOT so backup_root behaves predictably in containers.

docker-compose.yml

@@ -13,6 +13,7 @@ services:
     volumes:
       - pobsync_state:/opt/pobsync
       - pobsync_db:/var/lib/pobsync
+      - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
 
   scheduler:
     build: .
@@ -26,6 +27,7 @@ services:
     volumes:
       - pobsync_state:/opt/pobsync
       - pobsync_db:/var/lib/pobsync
+      - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
 
   web-mariadb:
     profiles: ["mariadb"]
@@ -48,6 +50,7 @@ services:
       - "8010:8000"
     volumes:
       - pobsync_state:/opt/pobsync
+      - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
 
   scheduler-mariadb:
     profiles: ["mariadb"]
@@ -68,6 +71,7 @@ services:
         condition: service_healthy
     volumes:
       - pobsync_state:/opt/pobsync
+      - ${POBSYNC_BACKUP_ROOT:-./backups}:/backups
 
   db:
     profiles: ["mariadb"]