Make manage.py use the service's database

manage.py only found the database via BACKUP_DASH_DB, which is set in
the systemd unit and not in an admin's shell, so it silently created a
new backup-dashboard.db in the current directory. API keys created
there were never seen by the service.

- Settings now come from /etc/backup-dashboard/backup-dashboard.env,
  which both the systemd unit (EnvironmentFile=) and manage.py read;
  real environment variables still take precedence
- manage.py prints the database it uses and refuses to create a new
  one unless --create is passed; --db overrides the path

Co-Authored-By: Claude Opus 5.5 <[email protected]>

deploy/backup-dashboard.env

@@ -0,0 +1,4 @@
+# Install as /etc/backup-dashboard/backup-dashboard.env
+# Read by both the API service and manage.py, so they always use the same database.
+BACKUP_DASH_DB=/var/lib/backup-dashboard/backup-dashboard.db
+BACKUP_DASH_RETENTION_DAYS=90