deps: declare rexml — no longer a default gem on Ruby 3.4, aws-sdk needs it

Gemfile

@@ -19,6 +19,9 @@ gem 'prawn-table',     '~> 0.2'
 gem 'rqrcode',         '~> 2.2'
 gem 'chunky_png',      '~> 1.4'
 gem 'aws-sdk-s3',      '~> 1.170'
+# aws-sdk-core needs an XML parser; rexml stopped being a default gem in
+# Ruby 3.4, so it must be declared or S3 calls raise at first use.
+gem 'rexml',           '~> 3.4'
 
 group :development do
   gem 'rerun', '~> 0.14'

Gemfile.lock

@@ -94,6 +94,7 @@ GEM
       ffi (~> 1.0)
     rerun (0.14.0)
       listen (~> 3.0)
+    rexml (3.4.4)
     rqrcode (2.2.0)
       chunky_png (~> 1.0)
       rqrcode_core (~> 1.0)
@@ -146,6 +147,7 @@ DEPENDENCIES
   rack-session (~> 2.1)
   rackup (~> 2.3)
   rerun (~> 0.14)
+  rexml (~> 3.4)
   rqrcode (~> 2.2)
   sequel (~> 5.86)
   sinatra (~> 4.2)