Cache-bust app JS/CSS on each deploy

Browsers (and a caching proxy in front of production) were holding
onto /js and /css for hours, so a deploy could go unnoticed by an
already-open tab. A boot-time version query string on the entry
script/stylesheet, plus an import map remapping map.js's internal
module imports to the same versioned URLs, fixes that without editing
every JS file's import statements.

app.rb

@@ -20,6 +20,14 @@ module Maptool
     set :sessions, httponly: true, secure: production?, same_site: :lax,
                    secret: ENV.fetch("SESSION_SECRET") { SecureRandom.hex(32) }
 
+    # Cache-busts /js and /css requests: browsers (and a caching proxy in
+    # front of production, see docs/deployment.adoc) hang onto static
+    # assets for hours, so without this a deploy that changes app.js/css
+    # can go unnoticed by an already-open tab until that cache expires.
+    # A fresh value every process start (i.e. every deploy, which restarts
+    # the container) is enough — it doesn't need to survive restarts.
+    ASSET_VERSION = Time.now.to_i
+
     OFFLINE_PMTILES_PATH = File.join(__dir__, "data", "offline.pmtiles")
     GPX_STORE = GpxStore.new(File.join(__dir__, "data", "gpx"))
     LIVETRACK_STORE = LiveTrackStore.new(File.join(__dir__, "data", "livetrack.json"))