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_host2.conf

@@ -14,7 +14,7 @@
 #
 # ── STATUS ────────────────────────────────────────────────────────────────────────────────────
 # HOST2 is currently being rebuilt — most sections scaffolded, fill in when back online.
-# When ready: set FAILOVER_ENABLED=true and DAILY_RSYNC_ENABLED=true in master.conf.
+# When ready: set FALLBACK_ENABLED=true and DAILY_RSYNC_ENABLED=true in master.conf.
 #
 # ── INDEX ─────────────────────────────────────────────────────────────────────────────────────
 #
@@ -137,7 +137,7 @@
     HOST2_WEEKLY_SYNC_SHARES=(
         # fill in when HOST2 is back online
         # "/mnt/user/Media_Server/Emby"
-        # "/mnt/user/appdata-Failover/Critical-Data"
+        # "/mnt/user/appdata-Fallback/Critical-Data"
     )
 
 # ━━━ Critical Sync Shares ━━━
@@ -145,8 +145,8 @@
 # Format: "/path/to/share" or "/path/to/share|profile-name"
     HOST2_CRITICAL_SYNC_SHARES=(
         # fill in when HOST2 is back online
-        # "/mnt/user/appdata-Failover/Critical-Data|critical-failover"
-        # "/mnt/user/Media_Server/Emby|emby-failover"
+        # "/mnt/user/appdata-Fallback/Critical-Data|critical-fallback"
+        # "/mnt/user/Media_Server/Emby|emby-fallback"
     )
 
 # ━━━ Backup Verify ━━━
@@ -161,7 +161,7 @@
 # HOST2-specific appdata sync profile — extends the shared PROFILE_* arrays in master.conf.
 # Use for appdata unique to HOST2.
 # Shared appdata (auth stack, Emby) use dedicated profiles defined in master.conf.
-# Run manually: bash Rsync/rsync.sh /mnt/user/appdata-Failover/HOST2-Appdata --profile=host2-appdata
+# Run manually: bash Rsync/rsync.sh /mnt/user/appdata-Fallback/HOST2-Appdata --profile=host2-appdata
     PROFILE_RSYNC_OPTS[host2-appdata]="-av --info=progress2 --bwlimit=${PROFILE_BW_LIMIT[host2-appdata]:-8000}"
     PROFILE_BW_LIMIT[host2-appdata]=8000
     PROFILE_RETRY_COUNT[host2-appdata]=3
@@ -245,7 +245,7 @@
 # ==============================================================================================
 
 # ━━━ DDNS ━━━
-# DDNS containers HOST2 manages — started/stopped by failover.sh per DDNS absolute rules:
+# DDNS containers HOST2 manages — started/stopped by fallback.sh per DDNS absolute rules:
 #   Internet loss → stop immediately
 #   Failover → HOST1 starts HOST2's DDNS as Tier 1 (before any other containers)
 #   Handback → stop HOST2's DDNS on HOST1 → rsync → start containers → start local DDNS last
@@ -256,15 +256,15 @@
 # ━━━ Internet Loss ━━━
 # Containers stopped immediately on HOST2 when internet connection is lost.
 # Prevents external-facing services from operating without connectivity.
-    FAILOVER_HOST2_STOP_ON_NO_NET=(
+    FALLBACK_HOST2_STOP_ON_NO_NET=(
         "Gmer4Lfe.us"
     )
 
-# ━━━ Failover Tiers — HOST2 Runs for HOST1 ━━━
+# ━━━ Fallback Tiers — HOST2 Runs for HOST1 ━━━
 # Containers HOST2 starts when HOST1 goes down.
 # Tier 1 is always immediate — vital services cannot wait.
 # Higher tiers activate after HOST1_TIER*_DELAY minutes (set in master_host1.conf).
-    FAILOVER_HOST2_RUNS_FOR_HOST1_TIER1=(
+    FALLBACK_HOST2_COVERS_HOST1_TIER1=(
         "Gmer4Lfe.com"
         "Emby"
         "VaultWarden-Gmer4Lfe"
@@ -274,18 +274,18 @@
         "ErsatzTV-Emby"
     )
 
-    FAILOVER_HOST2_RUNS_FOR_HOST1_TIER2=(
+    FALLBACK_HOST2_COVERS_HOST1_TIER2=(
         "Postgres-NextCloud"
         "NextCloud"
         "PostgreSQL_Immich"
         "Immich-Gmer4Lfe"
     )
 
-    FAILOVER_HOST2_RUNS_FOR_HOST1_TIER3=(
+    FALLBACK_HOST2_COVERS_HOST1_TIER3=(
         "Gitea"
     )
 
-    FAILOVER_HOST2_RUNS_FOR_HOST1_TIER4=(
+    FALLBACK_HOST2_COVERS_HOST1_TIER4=(
         "Sonarr"
         "Radarr"
         "Lidarr"
@@ -315,20 +315,20 @@
     HOST2_TIER1_WRITEBACK_DELAY=60  # skip writeback if outage under 1hr
 
 # Tier 4 automatically syncs HOST2_DAILY_SYNC_SHARES — only list paths NOT in that array.
-    FAILOVER_HOST2_WRITEBACK_TIER1=(
-        # "/mnt/user/appdata-Failover/Jayred365-Emby"
+    FALLBACK_HOST2_WRITEBACK_TIER1=(
+        # "/mnt/user/appdata-Fallback/Jayred365-Emby"
     )
 
-    FAILOVER_HOST2_WRITEBACK_TIER2=(
-        # "/mnt/user/appdata-Failover/Jayred365-Important"
+    FALLBACK_HOST2_WRITEBACK_TIER2=(
+        # "/mnt/user/appdata-Fallback/Jayred365-Important"
     )
 
-    FAILOVER_HOST2_WRITEBACK_TIER3=(
+    FALLBACK_HOST2_WRITEBACK_TIER3=(
         # "location-placeholder"
     )
 
-    FAILOVER_HOST2_WRITEBACK_TIER4=(
-        "/mnt/user/appdata-Failover/Arrs_Stack"     # arr databases — downloads queued during outage
+    FALLBACK_HOST2_WRITEBACK_TIER4=(
+        "/mnt/user/appdata-Fallback/Arrs_Stack"     # arr databases — downloads queued during outage
     )
 
 # ==============================================================================================