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>

master_host1.conf

@@ -105,7 +105,7 @@
 # Paths HOST2 should collect during the grace window after offboard.
 # Notified on offboard — no auto-deletion, HOST2 must collect manually within PARTNERSHIP_GRACE_HOURS.
     HOST1_PARTNERSHIP_MIRROR_BACKUPS=(
-        # "/mnt/user/appdata-Failover/Jayred365-Emby"
+        # "/mnt/user/appdata-Fallback/Jayred365-Emby"
     )
 
 # ==============================================================================================
@@ -148,7 +148,7 @@
 # Order matters — Emby first (larger transfer), then Critical-Data (auth stack).
     HOST1_WEEKLY_SYNC_SHARES=(
         "/mnt/user/Media_Server/Emby"               # emby profile — full clean mirror
-        "/mnt/user/appdata-Failover/Critical-Data"  # critical-data profile — auth stack
+        "/mnt/user/appdata-Fallback/Critical-Data"  # critical-data profile — auth stack
     )
 
 # ━━━ Critical Sync Shares ━━━
@@ -156,8 +156,8 @@
 # Format: "/path/to/share" or "/path/to/share|profile-name"
 # Order matters — Critical-Data first (auth stack), then Emby dirty sync.
     HOST1_CRITICAL_SYNC_SHARES=(
-        "/mnt/user/appdata-Failover/Critical-Data|critical-failover"  # auth dirty sync — stays running
-        "/mnt/user/Media_Server/Emby|emby-failover"                   # Emby dirty sync — stays running
+        "/mnt/user/appdata-Fallback/Critical-Data|critical-fallback"  # auth dirty sync — stays running
+        "/mnt/user/Media_Server/Emby|emby-fallback"                   # Emby dirty sync — stays running
     )
 
 # ━━━ Backup Verify ━━━
@@ -172,7 +172,7 @@
 # HOST1-specific appdata sync profile — extends the shared PROFILE_* arrays in master.conf.
 # Use for appdata unique to HOST1 (Organizrv2, VaultWarden, UptimeKuma etc.)
 # Shared appdata (auth stack, Emby) use dedicated profiles defined in master.conf.
-# Run manually: bash Rsync/rsync.sh /mnt/user/appdata-Failover/HOST1-Appdata --profile=host1-appdata
+# Run manually: bash Rsync/rsync.sh /mnt/user/appdata-Fallback/HOST1-Appdata --profile=host1-appdata
     PROFILE_RSYNC_OPTS[host1-appdata]="-av --info=progress2 --bwlimit=${PROFILE_BW_LIMIT[host1-appdata]:-8000}"
     PROFILE_BW_LIMIT[host1-appdata]=8000
     PROFILE_RETRY_COUNT[host1-appdata]=3
@@ -282,7 +282,7 @@
 # ==============================================================================================
 
 # ━━━ DDNS ━━━
-# DDNS containers HOST1 manages — started/stopped by failover.sh per DDNS absolute rules:
+# DDNS containers HOST1 manages — started/stopped by fallback.sh per DDNS absolute rules:
 #   Internet loss → stop immediately
 #   Failover → HOST2 starts HOST1's DDNS as Tier 1 (before any other containers)
 #   Handback → stop HOST1's DDNS on HOST2 → rsync → start containers → start local DDNS last
@@ -293,28 +293,28 @@
 # ━━━ Internet Loss ━━━
 # Containers stopped immediately on HOST1 when internet connection is lost.
 # Prevents external-facing services from operating without connectivity.
-    FAILOVER_HOST1_STOP_ON_NO_NET=(
+    FALLBACK_HOST1_STOP_ON_NO_NET=(
         "Gmer4Lfe.com"
     )
 
-# ━━━ Failover Tiers — HOST1 Runs for HOST2 ━━━
+# ━━━ Fallback Tiers — HOST1 Runs for HOST2 ━━━
 # Containers HOST1 starts when HOST2 goes down.
 # Tier 1 is always immediate — vital services cannot wait.
 # Higher tiers activate after HOST2_TIER*_DELAY minutes (set in master_host2.conf).
-    FAILOVER_HOST1_RUNS_FOR_HOST2_TIER1=(
+    FALLBACK_HOST1_COVERS_HOST2_TIER1=(
         "Gmer4Lfe.us"
         "VaultWarden-Jayred365"
     )
 
-    FAILOVER_HOST1_RUNS_FOR_HOST2_TIER2=(
+    FALLBACK_HOST1_COVERS_HOST2_TIER2=(
         # "container-placeholder"
     )
 
-    FAILOVER_HOST1_RUNS_FOR_HOST2_TIER3=(
+    FALLBACK_HOST1_COVERS_HOST2_TIER3=(
         # "container-placeholder"
     )
 
-    FAILOVER_HOST1_RUNS_FOR_HOST2_TIER4=(
+    FALLBACK_HOST1_COVERS_HOST2_TIER4=(
         # "container-placeholder"
     )
 
@@ -334,20 +334,20 @@
     HOST1_TIER1_WRITEBACK_DELAY=60  # skip Emby writeback if outage under 1hr
 
 # Tier 4 automatically syncs HOST1_DAILY_SYNC_SHARES — only list paths NOT in that array.
-    FAILOVER_HOST1_WRITEBACK_TIER1=(
+    FALLBACK_HOST1_WRITEBACK_TIER1=(
         "/mnt/user/Media_Server/Emby"               # watch states built up during outage
     )
 
-    FAILOVER_HOST1_WRITEBACK_TIER2=(
-        "/mnt/user/appdata-Failover/Important-Data" # NextCloud + Postgres — files added during outage
+    FALLBACK_HOST1_WRITEBACK_TIER2=(
+        "/mnt/user/appdata-Fallback/Important-Data" # NextCloud + Postgres — files added during outage
     )
 
-    FAILOVER_HOST1_WRITEBACK_TIER3=(
+    FALLBACK_HOST1_WRITEBACK_TIER3=(
         # "location-placeholder"
     )
 
-    FAILOVER_HOST1_WRITEBACK_TIER4=(
-        "/mnt/user/appdata-Failover/Arrs_Stack"     # arr databases — downloads queued during outage
+    FALLBACK_HOST1_WRITEBACK_TIER4=(
+        "/mnt/user/appdata-Fallback/Arrs_Stack"     # arr databases — downloads queued during outage
     )
 
 # ==============================================================================================
@@ -576,7 +576,7 @@
 
 # /boot read-only detected → reboot immediately.
 # Unexpected read-only /boot means state files and config writes are silently failing.
-# Failover state, watchdog reboot log, and lock files all go stale silently.
+# Fallback state, watchdog reboot log, and lock files all go stale silently.
     HOST1_SYS_WATCHDOG_CHECK_BOOT=true
 
 # ━━━ Tier 2 — Urgent OOM Check ━━━