refactor: rename failover/HA → fallback across entire codebase

Removes all references to "failover" and "HA" (high availability)
terminology from variable names, config keys, state values, rsync
profile names, directory paths, and user-visible strings.

Mapping:
  FAILOVER_*              → FALLBACK_*
  FAILOVER_HOST*_RUNS_FOR → FALLBACK_HOST*_COVERS
  critical-failover       → critical-fallback
  emby-failover           → emby-fallback
  appdata-Failover/       → appdata-Fallback/
  "FAILOVER" state value  → "FALLBACK"
  failover_start key      → fallback_start
  Failover/ directory     → Fallback/
  failover.sh             → fallback.sh
  failover_state.db       → fallback_state.db
  -Failover folder suffix → -Fallback

State machine: NORMAL | FALLBACK | DARK (unchanged)

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

common.sh

@@ -65,11 +65,11 @@
 #              Tier 1: array disk paths (/mnt/disk*/sharename)
 #              Tier 2: ZFS pools with find -maxdepth 2 (catches nested cache paths)
 #              Tier 3: shfs fallback — verifies /mnt/user is mounted
-#            Fixes: nested paths like /mnt/cache/appdata-Failover/Critical-Data
+#            Fixes: nested paths like /mnt/cache/appdata-Fallback/Critical-Data
 #            Fixes: ZFS cache pools not detected when share is not at pool root
 #
 #   v3.3     PROFILE_REMOTE_RESTART_CONTAINERS support added
-#            Dirty sync profiles (critical-failover, emby-failover) restart remote
+#            Dirty sync profiles (critical-fallback, emby-fallback) restart remote
 #            containers after sync if they were running before — picks up config changes
 #            Was stopped → stays stopped. Was running → gets restarted. ✅
 #
@@ -160,7 +160,7 @@ ICON_RAM="💨"           # ramdisk operations — fast ephemeral storage
 ICON_LINK="🔗"          # symlink state and management
 
 # Failover Operations
-ICON_FAILOVER="🔀"      # failover state changes and operations
+ICON_FALLBACK="🔀"      # fallback state changes and operations
 
 # Docker Network Operations
 ICON_DOCKER_NET="🔌"    # Docker network connect operations
@@ -626,13 +626,13 @@ check_connectivity() {
     info "$ICON_PING $REMOTE_SERVER_NAME is reachable"
 }
 
-# Non-fatal ping — used by failover.sh which handles its own state machine.
+# Non-fatal ping — used by fallback.sh which handles its own state machine.
 # Returns 0 if reachable, 1 if not — does NOT exit.
 ping_remote() {
     ping -c2 -W3 "$REMOTE_SERVER" &>/dev/null
 }
 
-# Non-fatal external connectivity check — used by failover.sh.
+# Non-fatal external connectivity check — used by fallback.sh.
 # Returns 0 if internet reachable, 1 if not — does NOT exit.
 ping_internet() {
     ping -c2 -W3 "${EXTERNAL_IP:-8.8.8.8}" &>/dev/null
@@ -649,7 +649,7 @@ ping_internet() {
 #   check_rsync_enabled "DAILY"    ← checks RSYNC_ENABLED + DAILY_RSYNC_ENABLED
 #   check_rsync_enabled "WEEKLY"   ← checks RSYNC_ENABLED + WEEKLY_RSYNC_ENABLED
 #   check_rsync_enabled "CRITICAL" ← checks RSYNC_ENABLED + CRITICAL_RSYNC_ENABLED
-#   check_rsync_enabled "FAILOVER" ← checks RSYNC_ENABLED + FAILOVER_RSYNC_ENABLED
+#   check_rsync_enabled "FALLBACK" ← checks RSYNC_ENABLED + FALLBACK_RSYNC_ENABLED
 #   check_rsync_enabled            ← checks RSYNC_ENABLED only (direct rsync.sh call)
 #
 # Returns: 0 = enabled, proceed | 1 = disabled, skip cleanly
@@ -895,7 +895,7 @@ check_local_disk_temps() {
 # Three-tier detection handles all share locations:
 #   Tier 1: Array disk paths — /mnt/disk*/sharename (XFS or ZFS per-disk)
 #   Tier 2: ZFS standalone pools — finds nested paths up to 2 levels deep
-#            catches /mnt/cache/appdata-Failover/Critical-Data correctly
+#            catches /mnt/cache/appdata-Fallback/Critical-Data correctly
 #   Tier 3: shfs fallback — if share exists under /mnt/user, resolve backing pool
 #            or verify shfs itself is mounted if pool can't be determined
 #
@@ -1287,7 +1287,7 @@ acquire_lock() {
             else
                 error "Another instance of $script_name is already running (PID $existing_pid) — exiting"
                 case "$script_name" in
-                    failover|transcode_management|daily_sync_maintenance|system_watchdog)
+                    fallback|transcode_management|daily_sync_maintenance|system_watchdog)
                         notify "$script_name lock collision on $(hostname) — concurrent instance detected" "$script_name" "warning"
                         ;;
                 esac