Fix Planetiler tmp-file format detection, add build progress and delete

Root cause of "planetiler exited with status 1: ... Unsupported format
tmp" (and likely the earlier unexplained "pmtiles exited with status
1"): Planetiler/go-pmtiles pick their output archive format from the
file extension, and the atomic-rename tmp files were named
"offline.pmtiles.tmp"/"terrain.pmtiles.tmp" — ".tmp" as the extension,
not ".pmtiles". Renamed to "offline.tmp.pmtiles"/"terrain.tmp.pmtiles".

Also, since build-status only ever said "running since X" with no way
to tell what it was actually doing:
- GET .../build-status now includes a `step` string ("fetching PL
  (3/4) — done: SK, CZ — pending: HU", "tiling with planetiler", ...)
  while a build is in progress.
- DELETE /api/offline-tiles / /api/terrain-tiles clear a built file,
  its sidecar meta, and (for offline) the whole per-country download
  cache — a manual "reclaim space" / "start over" action the region
  picker's Apply never did on its own. DELETE
  /api/offline-tiles/cache/:code clears just one country's cached
  extract. All three are 409 while that kind's build is running, and
  surfaced as a small "Delete" button on each row in the Offline data
  panel.

app.rb

@@ -277,6 +277,33 @@ module Maptool
       json PmtilesBuilder.offline_status
     end
 
+    # Deletes offline.pmtiles plus its whole per-country download cache —
+    # a manual "reclaim the space"/"start over" action, distinct from
+    # unticking countries in the region picker (which only changes what a
+    # future build covers, doesn't free anything already on disk).
+    delete "/api/offline-tiles" do
+      require_login!
+      begin
+        PmtilesBuilder.delete_offline_data(DATA_DIR)
+      rescue PmtilesBuilder::BuildError => e
+        halt 409, json(error: e.message)
+      end
+      json ok: true
+    end
+
+    # Deletes one country's cached Geofabrik extract only — e.g. to force a
+    # clean re-download of just that one, or reclaim space for a country
+    # that's no longer selected (the cache otherwise keeps it indefinitely).
+    delete "/api/offline-tiles/cache/:code" do
+      require_login!
+      begin
+        PmtilesBuilder.delete_cached_extract(DATA_DIR, params[:code])
+      rescue PmtilesBuilder::BuildError => e
+        halt 409, json(error: e.message)
+      end
+      json ok: true
+    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.
@@ -309,6 +336,17 @@ module Maptool
       json PmtilesBuilder.terrain_status
     end
 
+    # Same reasoning as DELETE /api/offline-tiles above.
+    delete "/api/terrain-tiles" do
+      require_login!
+      begin
+        PmtilesBuilder.delete_terrain_data(DATA_DIR)
+      rescue PmtilesBuilder::BuildError => e
+        halt 409, json(error: e.message)
+      end
+      json ok: true
+    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