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.

common.sh

@@ -1923,4 +1923,10 @@ show_status() {
     echo "Delayed:      ${DELAYED_CONTAINERS[*]:-n/a}"
     echo "Excludes:     ${EXCLUDE_DIRS[*]:-n/a}"
     echo "━━━━━━━━━━━━━━━━━━━━━━━"
+}
+
+require_partnership() {
+    [[ "${PARTNERSHIP_ENABLED:-false}" == "true" ]] && return 0
+    log "PARTNERSHIP_ENABLED=false — skipping cross-server operation"
+    exit 0
 }