Back up existing files before every service overwrites them

Confirmed live: install_frigate()'s fresh-install path overwrote a
working, hand-crafted docker-compose.yml (Frigate + mosquitto +
frigate-notify) with zero backup, because that file's shape didn't match
what frigate.sh's own "existing install" detection knew how to recognize.
Every service's own detection is a judgment call about what counts as
"already installed" and can miss a real setup built outside this repo's
conventions.

lib/common.sh gains backup_if_exists(FILE) — copies FILE to
FILE.bak.<timestamp> if it exists, no-ops otherwise (including DRY_RUN).
Applied before every service's own `cat > docker-compose.yml`/`cat > .env`
write across all 60 services that do one (115 call sites), plus a matching
standalone-mode stub added to every service's own bootstrap block, same
convention already used for port_in_use/find_free_port. This doesn't
replace a service's own update/fresh-reinstall detection — it's the safety
net underneath it, so a wrong detection costs a .bak file to restore from
instead of the original silently disappearing.

Also fixes the actual gap that surfaced this: services/frigate.sh's
Authelia offer only checked for Authelia installed locally on Frigate's
own box, which is never true for a dedicated NVR box with no local Caddy
either (the common shape — Caddy lives elsewhere, snippet-generation mode
already handles that). Now offers Authelia protection unconditionally and,
when Authelia isn't local, asks whether it lives on the same machine as
Caddy (still "import authelia", since that's local to wherever Caddy ends
up) or on a genuinely separate third machine (the explicit
header-pinned forward_auth form, per CLAUDE.md's "forward_auth to a remote
Authelia" note, needed because a bare authelia:9091 shortcut only works
one hop).

services/meshcentral.sh

@@ -188,6 +188,11 @@ CBLOCK
             mkdir -p "$_dir"
             cat > "$_dir/README.md"
         }
+        backup_if_exists() {
+            local _file="$1"
+            [ -f "$_file" ] || return 0
+            cp -p "$_file" "${_file}.bak.$(date +%Y%m%d-%H%M%S)" 2>/dev/null
+        }
     fi
 
     # Globals — ACTUAL_USER/ACTUAL_HOME must come before DOCKER_DIR
@@ -323,6 +328,7 @@ networks:
 "
     fi
 
+    backup_if_exists docker-compose.yml
     cat > docker-compose.yml << MC_COMPOSE
 name: $CONTAINER
 
@@ -350,6 +356,7 @@ services:
 ${_CADDY_NET_BLOCK}${_CADDY_NET_SECTION}
 MC_COMPOSE
 
+    backup_if_exists .env
     cat > .env << MC_ENV
 MC_HOSTNAME=$MC_HOSTNAME
 MC_REVERSE_PROXY=false