Adds require_partnership() to common.sh — exits cleanly when PARTNERSHIP_ENABLED=false.
Removes FALLBACK_PARTNERSHIP_REQUIRED toggle — partnership is now always required,
not optional. Cross-server scripts (rsync, conf sync, fallback, arr sync, play state,
backup verify) all call require_partnership after detect_hosts.
Writes partner confs from RAM cache to /boot/config/.cache/vv/d/ while
remote is offline, and removes the backup when remote comes back. Called
each minute via SYSTEM_WATCHDOG_SCRIPTS so crashes and power loss are
covered — not just graceful shutdowns.