Follow Geofabrik's 302 redirects on extract download and Last-Modified check

Geofabrik redirects europe/<slug>-latest.osm.pbf to a dated filename
(confirmed live: .../europe/austria-latest.osm.pbf -> .../europe/austria-
260910.osm.pbf) — a real build hit this immediately. Net::HTTP doesn't
follow redirects on its own; both the streaming download and the HEAD-based
update-check now do, up to a small limit.

lib/maptool/pmtiles_builder.rb

@@ -262,16 +262,35 @@ module Maptool
     private_class_method :geofabrik_url
 
     # Real network implementation of the download_extract seam above —
-    # streams the extract to dest and returns its Last-Modified header
+    # streams the extract to dest (some of these are multi-GB — Net::HTTP's
+    # plain #get buffers the whole response body in memory first, which
+    # read_body's block form avoids) and returns its Last-Modified header
     # (free off the same GET, no separate HEAD needed) for the sidecar
-    # meta / update-check below.
-    def self.download_extract(url, dest)
-      uri = URI(url)
-      response = Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == "https") { |http| http.get(uri) }
-      raise BuildError, "could not download #{url} (HTTP #{response.code})" unless response.is_a?(Net::HTTPSuccess)
+    # meta / update-check below. Geofabrik 302s (e.g. .../europe/austria
+    # redirecting to wherever actually serves it) — followed manually
+    # since request_get's block form doesn't do this itself.
+    def self.download_extract(url, dest, limit: 5)
+      raise BuildError, "too many redirects downloading #{url}" if limit.zero?
 
-      File.binwrite(dest, response.body)
-      response["last-modified"]
+      uri = URI(url)
+      last_modified = nil
+      redirect_to = nil
+      Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == "https") do |http|
+        http.request_get(uri) do |response|
+          case response
+          when Net::HTTPSuccess
+            last_modified = response["last-modified"]
+            File.open(dest, "wb") { |file| response.read_body { |chunk| file.write(chunk) } }
+          when Net::HTTPRedirection
+            redirect_to = URI.join(url, response["location"]).to_s
+          else
+            raise BuildError, "could not download #{url} (HTTP #{response.code})"
+          end
+        end
+      end
+      return download_extract(redirect_to, dest, limit: limit - 1) if redirect_to
+
+      last_modified
     end
 
     # Sidecar recording which Geofabrik sources (country code, slug, and
@@ -309,12 +328,16 @@ module Maptool
       nil
     end
 
-    def self.fetch_geofabrik_last_modified(url)
+    def self.fetch_geofabrik_last_modified(url, limit: 5)
+      raise BuildError, "too many redirects reaching #{url}" if limit.zero?
+
       uri = URI(url)
       response = Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == "https") { |http| http.head(uri) }
-      raise BuildError, "could not reach #{url}" unless response.is_a?(Net::HTTPSuccess)
-
-      response["last-modified"]
+      case response
+      when Net::HTTPSuccess then response["last-modified"]
+      when Net::HTTPRedirection then fetch_geofabrik_last_modified(URI.join(url, response["location"]).to_s, limit: limit - 1)
+      else raise BuildError, "could not reach #{url}"
+      end
     end
 
     def self.offline_meta_path(data_dir)