Convert GPX/FIT to GeoJSON at upload time, not first view

GpxStore#save now calls geojson() itself right after writing the
file, so the (potentially tens-of-seconds, for a large POI export)
conversion cost is paid during the upload — covered by the frontend's
existing "Uploading…" status — instead of on whichever checkbox
toggle happens to be first, which otherwise reads as a silent hang
indistinguishable from broken. An unparseable file still saves fine;
the error just surfaces on first view same as before.

docs/architecture.adoc

@@ -269,8 +269,14 @@ know which format it got. `GpxStore#geojson` caches that conversion
 alongside the source file (`<path>.geojson`, invalidated by mtime) —
 REXML parsing a several-thousand-waypoint POI export (a park4night
 area export, say) can take tens of seconds, and without a cache that
-cost repeats on every single toggle rather than just the first one
-after a save. `Gpx` parses the GPX with REXML: `<wpt>`
+cost repeats on every single toggle. `GpxStore#save` also calls
+`geojson` itself right after writing the file, so that cost is paid
+once during the upload (covered by the frontend's own "Uploading…"
+status) instead of on whichever toggle happens to be first — a silent
+multi-second hang on a checkbox click reads as broken in a way the
+same wait during an upload doesn't. A file that fails to parse still
+saves fine either way; the error just surfaces on first view instead.
+`Gpx` parses the GPX with REXML: `<wpt>`
 waypoints become `Point` features (with `name`, `desc` and `time`
 carried through as properties), each `<trk>` becomes one
 `MultiLineString` feature (one line per `<trkseg>`), and each `<rte>`