Bake cross-service port collision avoidance into every service script

With 70+ services sharing a handful of common default ports (emby and
jellyfin both default to 8096, changedetection and frigate both default
to 5000, arm and nextcloud both default to 8080...), nothing previously
checked whether a service's default port was actually free on the host.
Whichever service installed second would silently write a compose file
claiming an already-held port, only failing at `docker compose up` time.

Adds two shared helpers to lib/common.sh:
- port_in_use PORT [PROTO] — true if something's already listening
- find_free_port VARNAME START [PROTO] — scans upward, writes back the
  first free port

Every service that publishes a fixed host port now scans before writing
docker-compose.yml, on every install (not just when adding an explicit
additional instance). On a normal single-install host this is a silent
no-op; it only changes behavior when something else already holds the
port.

- The 19 services already given multi-instance support this session had
  their port scan moved out of the "add instance" branch to run
  unconditionally, since the same collision risk exists on a plain first
  install.
- 20 more services with previously-hardcoded ports gained scanning for
  the first time: archivebox, arm, calibre-web, changedetection,
  drum-rhythm-game, gatus, n8n, nextcloud, onlyoffice, stirling-pdf,
  uptimekuma, portainer, iopaint (both GPU/CPU compose branches), koha
  (paired), syncthing (paired), wg-easy (paired, plus WG_PORT env so
  generated peer configs keep the right Endpoint), homeassistant
  (bridge-mode only — host mode can only warn), frigate and
  frigate-audio (multi-port stacks, moved together).
- caddy.sh is the deliberate exception: 80/443 stay fixed and only warn
  on collision, since silently moving Caddy itself would leave nothing
  listening where any client actually looks.
- authelia.sh needs no change — it has no published host port at all.
- Every service's standalone bootstrap fallback (sudo bash services/x.sh
  with no sibling files) got the same two helpers duplicated into its
  stub block, matching how every other shared helper is already handled
  there.

Documents the full pattern in CLAUDE.md's new "Port collision avoidance"
section, including the quoted-heredoc/backtick-escaping gotcha and the
network_mode:host limitation (can only scan ports the app takes as a
configurable env var).

Verified via bash -n on every changed file, plus functional runs seeding
occupied ports for each collision shape used here (single, paired,
multi-port stacks) and confirming the scan/shift and generated
compose/README output are correct — including the emby/jellyfin,
nextcloud/arm, and frigate/changedetection collision scenarios that
originally motivated this.

services/homeassistant.sh

@@ -44,6 +44,21 @@ if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
             chown -R "$ACTUAL_USER:$ACTUAL_USER" "$@" 2>/dev/null || true
         }
 
+        port_in_use() {
+            local _port="$1" _proto="${2:-tcp}"
+            local _flag="-tlnH"
+            [ "$_proto" = "udp" ] && _flag="-ulnH"
+            ss "$_flag" "sport = :${_port}" 2>/dev/null | grep -q .
+        }
+
+        find_free_port() {
+            local _varname="$1" _port="$2" _proto="${3:-tcp}"
+            while port_in_use "$_port" "$_proto"; do
+                _port=$((_port + 1))
+            done
+            eval "$_varname='$_port'"
+        }
+
         # Match common.sh's eval-based pattern so local vars in install_* are set correctly
         prompt_text() {
             local _q="$1" _def="$2" _var="$3" _r
@@ -189,9 +204,13 @@ install_homeassistant() {
     require_docker || return 1
     log_info "Installing Home Assistant..."
     local HOMEASSISTANT_DIR="$DOCKER_DIR/homeassistant"
+    local WEB_PORT="8123"
 
     if [ "$DRY_RUN" = true ]; then
         echo "[DRY-RUN] Would create $HOMEASSISTANT_DIR"
+        echo "[DRY-RUN] Bridge mode would auto-scan for a free host port; host mode"
+        echo "[DRY-RUN] can only warn on a collision (Home Assistant's own port isn't"
+        echo "[DRY-RUN] configurable, and host networking has no remapping)"
         return 0
     fi
 
@@ -224,16 +243,25 @@ install_homeassistant() {
         HA_NET_LINES="    network_mode: host"
         HA_CADDY_NET_LINES=""
         echo "  → Host networking selected (best device discovery)."
+        # Host mode has no port remapping and Home Assistant's own port isn't
+        # configurable via env var — a collision here can only be warned
+        # about, not silently fixed. See CLAUDE.md's "Port collision
+        # avoidance" section.
+        port_in_use "$WEB_PORT" && log_warning "Port $WEB_PORT is already in use by another process — Home Assistant won't be reachable until that's resolved (this port isn't configurable with host networking)."
     else
+        # Bridge mode can freely remap — scan for a free host port so a
+        # plain install doesn't silently claim a port another
+        # already-running service holds.
+        find_free_port WEB_PORT "$WEB_PORT"
         HA_NET_LINES="    ports:
-      - \"8123:8123\""
+      - \"${WEB_PORT}:8123\""
         if [ "$_CADDY_MODE" = "local" ]; then
             HA_CADDY_NET_LINES="    networks:
       - caddy_net"
         else
             HA_CADDY_NET_LINES=""
         fi
-        echo "  → Bridge networking selected (port 8123 published)."
+        echo "  → Bridge networking selected (port $WEB_PORT published)."
     fi
 
     local _CADDY_NET_SECTION=""
@@ -292,7 +320,7 @@ HA_CONFIG
     log_success "Home Assistant configured at $HOMEASSISTANT_DIR"
 
     if [ "$HA_NETMODE" = "2" ]; then
-        configure_caddy_for_service "Home Assistant" "8123" "home"
+        configure_caddy_for_service "Home Assistant" "$WEB_PORT" "home"
     else
         configure_caddy_for_service "Home Assistant" "homeassistant:8123" "home"
     fi
@@ -303,7 +331,7 @@ HA_CONFIG
 Home automation hub. Built-in auth — no Authelia needed.
 
 ## Access
-- URL: http://localhost:8123
+- URL: http://localhost:${WEB_PORT}
 - First run: create your admin account through the onboarding wizard
 
 ## Manage
@@ -322,7 +350,7 @@ MD
         docker compose up -d 2>/dev/null && log_success "Home Assistant started" || log_warning "Failed to start"
     fi
 
-    echo "  Access at:  http://localhost:8123"
+    echo "  Access at:  http://localhost:${WEB_PORT}"
     echo "  First run:  open the URL and create your admin account (onboarding)."
     echo "  Note:       first startup can take a minute while HA initializes."
     echo ""