Make PARTNERSHIP_ENABLED the authoritative gate for all cross-server operations

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.

Monitors/backup_verify.sh

@@ -124,6 +124,7 @@ acquire_lock
 
 # detect_hosts() sets MY_ID and aliases BACKUP_VERIFY_SHARES + DAILY_SYNC_SHARES
 detect_hosts
+require_partnership
 
 # Share selection — configured list or fallback to daily sync shares
 if [[ ${#BACKUP_VERIFY_SHARES[@]} -gt 0 ]]; then