feat: arr-native distributed media sync + config architecture fixes

Arr sync (new):
- Media/arr_sync.sh — full mesh bidirectional sync across all HOST* nodes
  - Lidarr (MusicBrainz), Sonarr (TVDB), Radarr (TMDB) all handled in one script
  - Remote API keys read live from config.xml via SSH — never stored in conf files
  - Shared blocklist (DATA_DIR/arr_sync_blocklist.tsv) merged from all nodes at runtime
  - Graceful skip if arr not configured locally or not reachable on a remote node
  - --blocklist-add / --blocklist-remove / --blocklist-list management flags
- daily_sync_maintenance.sh — arr sync runs as explicit phase before rsync
- partnership_onboard.sh — Step 3 bootstraps merged library on both sides at onboard
- master.conf — ARR_SYNC_* config block, DOCKER_APPDATA_BASE

Rsync / cleanup:
- DEFAULT_RSYNC_OPTS — removed --delete; arr_cleanup.sh owns orphan enforcement
- lidarr_cleanup.sh — removed HOST1-only guard; runs on any node with Lidarr configured

Config architecture:
- HOST1/HOST2 hostnames moved from master_host*.conf → master.conf (not credentials)
- Sparse checkout now works correctly: each server only needs its own host conf
- detect_hosts() still resolves MY_ID + REMOTE_ID via master.conf hostname values

Bug fix:
- common.sh line 493 — watchdog toggle eval had broken quoting; all SYS_WATCHDOG_CHECK_*
  globals were silently set to empty instead of their configured values

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

common.sh

@@ -490,7 +490,7 @@ detect_hosts() {
     for _wd_var in "${_wd_checks[@]}"; do
         local _wd_src="${MY_ID}_${_wd_var}"
         # Only alias if the host-specific var is set — preserves master.conf defaults
-        [[ -n "${!_wd_src+x}" ]] && eval "${_wd_var}="\${!_wd_src}""
+        [[ -n "${!_wd_src+x}" ]] && eval "${_wd_var}=\"\${${_wd_src}}\""
     done
 
     TRANSCODE_SSD_VAR="${MY_ID}_TRANSCODE_SSD"
@@ -782,6 +782,14 @@ check_remote_share() {
 # ── DISK TEMPERATURE CHECKS ───────────────────────────────────────────────────────────────────
 # ==============================================================================================
 
+# Returns 0 (true) if device is non-rotational (SSD/NVMe), 1 (false) if HDD.
+# Checks /sys/block/<device>/queue/rotational — 0=SSD, 1=HDD.
+is_ssd() {
+    local base
+    base=$(basename "$1")
+    [[ "$(cat "/sys/block/$base/queue/rotational" 2>/dev/null)" == "0" ]]
+}
+
 # Reads disk temperature thresholds from unRAID's dynamix.cfg.
 # Sets globals: UNRAID_DISK_HOT UNRAID_DISK_MAX UNRAID_SSD_HOT UNRAID_SSD_MAX
 # Falls back to values in master.conf (SMART_TEMP_WARN/CRIT) if file not found.