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>