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>

Monitors/weekly_health_digest.sh

@@ -20,7 +20,7 @@
 # master.conf to switch behaviour. No cron changes needed.
 #
 # ── DATA SOURCES (reads only) ─────────────────────────────────────────────────────────────────
-#   FAILOVER_STATE_FILE          — current failover state
+#   FALLBACK_STATE_FILE          — current fallback state
 #   SYS_WATCHDOG_FAILED_FILE     — container skip list (manual intervention needed)
 #   WATCHDOG_STATE_FILE          — active container watchdog strikes
 #   SYS_WATCHDOG_STATE_FILE      — active system watchdog strikes
@@ -44,7 +44,7 @@
 #   DIGEST_PROFILE            — always | smart | weekly
 #   DIGEST_DAY                — day name for weekly profile (e.g. Sunday)
 #   DIGEST_SMART_ON_WATCHDOG  — send on active watchdog strikes
-#   DIGEST_SMART_ON_FAILOVER  — send on non-NORMAL failover state
+#   DIGEST_SMART_ON_FALLBACK  — send on non-NORMAL fallback state
 #   DIGEST_SMART_ON_CERT_WARN — send on cert warning
 #   DIGEST_SMART_ON_BANDWIDTH — send on high bandwidth day
 #   CERT_WARN_DAYS / CERT_CRIT_DAYS / CERT_TIMEOUT
@@ -101,7 +101,7 @@ if [[ "$SHOW_STATUS" == true ]]; then
     echo "$ICON_HOST    Identity:       $MY_ID ($LOCAL_SERVER_NAME)"
     echo "$ICON_DIGEST  Profile:        $DIGEST_PROFILE"
     echo "$ICON_DIGEST  Digest day:     $DIGEST_DAY"
-    echo "$ICON_DIGEST  Smart triggers: watchdog=$DIGEST_SMART_ON_WATCHDOG failover=$DIGEST_SMART_ON_FAILOVER cert=$DIGEST_SMART_ON_CERT_WARN bandwidth=$DIGEST_SMART_ON_BANDWIDTH"
+    echo "$ICON_DIGEST  Smart triggers: watchdog=$DIGEST_SMART_ON_WATCHDOG fallback=$DIGEST_SMART_ON_FALLBACK cert=$DIGEST_SMART_ON_CERT_WARN bandwidth=$DIGEST_SMART_ON_BANDWIDTH"
     echo "$ICON_CERT    Cert domains:   ${CERT_MONITOR_DOMAINS[*]:-none}"
     echo "$ICON_BANDWIDTH Bandwidth warn: ${BANDWIDTH_WARN_GB}GB"
     echo "$ICON_GEAR    Dry Run:        $DRY_RUN"
@@ -148,17 +148,17 @@ ISSUES=()       # need attention
 DIGEST_LINES=() # full report lines
 
 # ── Failover State ────────────────────────────────────────────────────────────────────────────
-if [[ -f "$FAILOVER_STATE_FILE" ]]; then
-    FAILOVER_STATE=$(grep "^state=" "$FAILOVER_STATE_FILE" 2>/dev/null | cut -d= -f2)
-    if [[ -n "$FAILOVER_STATE" ]]; then
-        DIGEST_LINES+=("$ICON_FAILOVER Failover: $FAILOVER_STATE")
-        if [[ "$FAILOVER_STATE" != "NORMAL" ]]; then
-            ISSUES+=("Failover state: $FAILOVER_STATE")
-            [[ "$DIGEST_SMART_ON_FAILOVER" == true ]] && SHOULD_SEND=true
+if [[ -f "$FALLBACK_STATE_FILE" ]]; then
+    FALLBACK_STATE=$(grep "^state=" "$FALLBACK_STATE_FILE" 2>/dev/null | cut -d= -f2)
+    if [[ -n "$FALLBACK_STATE" ]]; then
+        DIGEST_LINES+=("$ICON_FALLBACK Fallback: $FALLBACK_STATE")
+        if [[ "$FALLBACK_STATE" != "NORMAL" ]]; then
+            ISSUES+=("Fallback state: $FALLBACK_STATE")
+            [[ "$DIGEST_SMART_ON_FALLBACK" == true ]] && SHOULD_SEND=true
         fi
     fi
 else
-    DIGEST_LINES+=("$ICON_FAILOVER Failover: state file not found")
+    DIGEST_LINES+=("$ICON_FALLBACK Fallback: state file not found")
 fi
 
 # ── Container Skip List ───────────────────────────────────────────────────────────────────────