Cache per-country Geofabrik extracts so Apply doesn't redownload everything

Every region-selection change re-downloaded every selected country's
.osm.pbf from scratch, even ones already fetched by a prior build —
painful for multi-GB countries. Extracts now persist in
data/geofabrik_cache/, keyed by country, and are only re-fetched when
Geofabrik's Last-Modified for that country actually moved on (checked
with a cheap HEAD) or the file isn't cached yet.

.gitignore

@@ -12,6 +12,10 @@
 # see docs/development.adoc)
 /data/*.pmtiles
 
+# Per-country .osm.pbf cache for offline.pmtiles rebuilds (also large,
+# also local-only — see PmtilesBuilder.cached_extract)
+/data/geofabrik_cache/
+
 # Offline Wikipedia ZIM for kiwix-serve (also large, also local-only —
 # same story, see docs/development.adoc)
 /data/kiwix/*