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.

Media/play_state_sync.sh

@@ -80,6 +80,7 @@ for _a in "$@"; do
     [[ "$_a" == "--full" ]] && FULL_SYNC=true || _FILTERED+=("$_a")
 done
 parse_args "${_FILTERED[@]}"
+require_partnership
 
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 # ━━━ Setup ━━━