Support uploading FIT files (and zipped exports) alongside GPX

Maptool::Fit reads Garmin's binary FIT format directly rather than
adding a gem — the one Ruby option depends on activesupport, unmaintained
since 2022, for a single-purpose binary parser. Only "record" messages
are read (position + altitude; everything else is skipped by byte
length, never interpreted); field numbers and scale/offset are
Garmin's own (fit-python-sdk's profile.py), and the framing plus all
extracted values were checked byte-for-byte against 5 real device
files from python-fitparse's test suite before this shipped.

A .zip upload (Garmin's bulk/FIT export format) is expanded into its
.gpx/.fit entries via rubyzip — that one's an actual dependency, since
ZIP's own format isn't the kind of narrow, single-purpose parsing FIT
above is.

Gemfile

@@ -5,6 +5,7 @@ gem "sinatra-contrib", "~> 4.2"
 gem "puma", "~> 8.0"
 gem "rexml", "~> 3.4"
 gem "oauth2", "~> 2.0"
+gem "rubyzip", "~> 2.4"
 
 group :development do
   gem "rerun"