Add in-app regeneration of offline.pmtiles and terrain.pmtiles

A logged-in user can now trigger, from a new "Offline data" sidebar
panel, what was previously a fully manual go-pmtiles workflow
(docs/development.adoc): regenerate either file for the current map
viewport, no area-picker (same "visible area is the area" convention
the Overpass POI plugin already uses).

- lib/maptool/pmtiles_builder.rb: two independent background jobs
  (Thread + Mutex-guarded status Hash, same shape as app.rb's own GNSS
  poller), each writing to a .tmp path and atomically renaming into
  place so a build never serves a partially-written file mid-run.
  offline.pmtiles is a single `pmtiles extract` against Protomaps'
  public daily basemap build (byte-range extraction, not a full
  ~120GB download). terrain.pmtiles has no equivalent public
  Terrarium-encoded archive to extract from, so it fetches individual
  Terrarium tiles itself, packs them into an intermediate MBTiles
  (new sqlite3 gem), then the same `pmtiles convert`.
- 4 new app.rb routes (POST .../build, GET .../build-status) plus
  size/mtime added to the existing availability endpoints.
- public/js/offline_data.js: new sidebar panel, polling build-status
  the same self-rescheduling-setTimeout shape livetrack.js already
  uses for its own refresh loop.
- Both network-heavy pieces (the Protomaps build index, per-tile
  fetches) are injectable, so test/test_pmtiles_builder.rb exercises
  the actual build pipeline against a fake `pmtiles` binary without
  hitting a real network, matching this suite's existing
  keep-it-offline convention for external services.

app.rb

@@ -12,6 +12,7 @@ require_relative "lib/maptool/livetrack_store"
 require_relative "lib/maptool/livetrack_client"
 require_relative "lib/maptool/kiwix"
 require_relative "lib/maptool/plugins"
+require_relative "lib/maptool/pmtiles_builder"
 
 module Maptool
   class App < Sinatra::Base
@@ -125,6 +126,12 @@ module Maptool
         halt 401, json(error: "login required", login_url: "/login") unless logged_in?
       end
 
+      def pmtiles_file_status(path)
+        return { available: false } unless File.exist?(path)
+
+        { available: true, size: File.size(path), mtime: File.mtime(path).utc.iso8601 }
+      end
+
       def callback_url
         "#{request.base_url}/auth/callback"
       end
@@ -204,7 +211,7 @@ module Maptool
     end
 
     get "/api/offline-tiles" do
-      json available: File.exist?(OFFLINE_PMTILES_PATH)
+      json pmtiles_file_status(OFFLINE_PMTILES_PATH)
     end
 
     # Served as a plain range-request-capable file, not decoded server-side:
@@ -216,11 +223,33 @@ module Maptool
       send_file OFFLINE_PMTILES_PATH, type: "application/octet-stream"
     end
 
+    # Regenerates offline.pmtiles for the given bbox (the current map
+    # viewport, same "visible area is the area" convention as the Overpass
+    # POI plugin) by extracting it from Protomaps' own daily full-planet
+    # basemap build — see lib/maptool/pmtiles_builder.rb. Runs in the
+    # background; poll build-status below for progress.
+    post "/api/offline-tiles/build" do
+      require_login!
+      bbox = %w[south west north east].map { |key| params[key].to_s }
+      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)
+
+      begin
+        PmtilesBuilder.build_offline(bbox.map(&:to_f), data_dir: DATA_DIR)
+      rescue PmtilesBuilder::BuildError => e
+        halt 409, json(error: e.message)
+      end
+      json status: "started"
+    end
+
+    get "/api/offline-tiles/build-status" do
+      json PmtilesBuilder.offline_status
+    end
+
     # Same optional-local-file story as offline.pmtiles above — see
     # docs/development.adoc for how to generate one, until then hillshade
     # just stays unavailable while the offline basemap is active.
     get "/api/terrain-tiles" do
-      json available: File.exist?(TERRAIN_PMTILES_PATH)
+      json pmtiles_file_status(TERRAIN_PMTILES_PATH)
     end
 
     get "/tiles/terrain.pmtiles" do
@@ -229,6 +258,23 @@ module Maptool
       send_file TERRAIN_PMTILES_PATH, type: "application/octet-stream"
     end
 
+    post "/api/terrain-tiles/build" do
+      require_login!
+      bbox = %w[south west north east].map { |key| params[key].to_s }
+      halt 400, json(error: "south, west, north, east are required") if bbox.any?(&:empty?)
+
+      begin
+        PmtilesBuilder.build_terrain(bbox.map(&:to_f), data_dir: DATA_DIR)
+      rescue PmtilesBuilder::BuildError => e
+        halt 409, json(error: e.message)
+      end
+      json status: "started"
+    end
+
+    get "/api/terrain-tiles/build-status" do
+      json PmtilesBuilder.terrain_status
+    end
+
     # Pinged once per page load (public/js/map.js) — url is a same-origin
     # path (not kiwix-serve's own address, which the browser can't reach
     # directly behind the reverse proxy in front of this app), proxied by