M2c-dev: development compose stack, backup job, Firebase setup docs

infra/dev: compose stack on the Mac (server kvitt-server:dev, on-demand backup),
.env.example, build/up/logs/backup/copy-google-services scripts. No Caddy, no TLS
(debug builds accept http to private LAN addresses, AC-2.8). Bind-mounted data.
infra/backup: alpine 3.24.2 image and backup.sh (SPEC 10.9; Drive upload skipped
when RCLONE_FOLDER_ID is empty). infra/README.md for the dev stack and Firebase.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

infra/dev/lib.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# Shared helpers for the infra/dev scripts. Sourced, not run.
+# shellcheck disable=SC2034  # variables are used by the scripts that source this file
+
+dev_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+repo_root="$(cd "$dev_dir/../.." && pwd)"
+
+# infra/dev/.env (git-ignored) when it exists, otherwise the committed example.
+if [[ -f "$dev_dir/.env" ]]; then env_file="$dev_dir/.env"; else env_file="$dev_dir/.env.example"; fi
+
+# Value of KEY in the env file (empty when unset or blank).
+env_value() {
+  sed -n "s/^$1=//p" "$env_file" | tail -n 1
+}
+
+compose() {
+  docker compose -f "$dev_dir/compose.yaml" --env-file "$env_file" "$@"
+}
+
+# Resolve a path from the env file the way compose does: relative to infra/dev/.
+dev_path() {
+  case "$1" in
+    /*) printf '%s\n' "$1" ;;
+    *) printf '%s\n' "$dev_dir/$1" ;;
+  esac
+}
+
+# The Mac's address on the interface that carries the default route (the home Wi-Fi).
+lan_address() {
+  local iface
+  iface="$(route -n get default 2>/dev/null | awk '/interface:/ {print $2}')"
+  [[ -n "$iface" ]] && ipconfig getifaddr "$iface" 2>/dev/null
+}
+
+dev_port() {
+  local p
+  p="$(env_value KVITT_DEV_PORT)"
+  printf '%s\n' "${p:-8080}"
+}