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.

lib/common.sh

@@ -542,6 +542,45 @@ write_readme() {
     chown "$ACTUAL_USER:$ACTUAL_USER" "$dir/README.md" 2>/dev/null || true
 }
 
+# ── Host port collision avoidance (shared by every service that publishes a
+# fixed host port) ────────────────────────────────────────────────────────────
+# With 70+ services in this repo, several ship the same default port (e.g.
+# emby and jellyfin both default to 8096; changedetection and frigate both
+# default to 5000). Nothing enforced those defaults were actually free on the
+# host, so whichever service started its container second would fail to bind
+# ("port is already allocated") instead of just landing on the next free port.
+# Confirmed live: installing jellyfin after emby (or vice versa) writes a
+# docker-compose.yml claiming a port the other service's container already
+# holds, and only fails at `docker compose up` time — not at install time.
+#
+# port_in_use PORT [PROTO]  — PROTO defaults to tcp; pass "udp" for UDP-only
+# ports. Returns 0 (true, in use) or 1 (free).
+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 VARNAME START [PROTO]
+# Scans upward from START for a free host port and writes the result back
+# into VARNAME. Single-port convenience wrapper around port_in_use — every
+# service's install_<name>() should run its default/candidate port through
+# this (or a hand-rolled port_in_use loop for multiple ports that must move
+# together, e.g. a web port + an agent port) before writing docker-compose.yml,
+# not only when adding an explicit additional instance of itself. On a normal
+# single-install host this is a silent no-op (the default port is free, so
+# VARNAME comes back unchanged); it only changes behavior when something else
+# already holds the port, which is exactly the case that used to fail at
+# startup instead of at install time.
+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'"
+}
+
 # ── Caddy reverse-proxy wiring (shared by every web service) ─────────────────
 # Usage: configure_caddy_for_service "Name" "UPSTREAM" "default-subdomain" ["extra"]
 # UPSTREAM: container:port for caddy_net routing (e.g. "filebrowser:80"),