Pass --download to Planetiler for its auxiliary OpenMapTiles sources

Real build failure once the tmp-filename fix got past the format-
detection error: "data/sources/lake_centerline.shp.zip does not exist.
Run with --download to fetch it". The OpenMapTiles profile needs a few
small non-OSM sources (water polygons, Natural Earth, lake
centerlines) alongside the .osm.pbf itself, and Planetiler only
fetches them when told to. They land under its own default
data/sources/, gitignored alongside the rest of data/ — fetched once,
reused by every later build.

.gitignore

@@ -16,6 +16,13 @@
 # also local-only — see PmtilesBuilder.cached_extract)
 /data/geofabrik_cache/
 
+# Planetiler's own auxiliary-source cache and scratch working directory
+# (water polygons/Natural Earth/lake centerlines, --download; temp feature
+# DBs, --tmpdir) — relative to the app's CWD, which lands them here
+# alongside the rest of data/ (see extract_offline's --download comment)
+/data/sources/
+/data/tmp/
+
 # Offline Wikipedia ZIM for kiwix-serve (also large, also local-only —
 # same story, see docs/development.adoc)
 /data/kiwix/*