Upload several Melee exports at once; let admins delete leagues

- The session import takes many files (every round's matches export plus
  the standings export) in one transaction; a bad file, a round repeated
  across files, or two standings exports rejects the whole batch.
- Delete league (with its sessions and results; players are kept).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

views/league.ejs

@@ -9,3 +9,4 @@
 <% if (admin) { %><form method="post" action="<%= base %>/admin/league/<%= lg.id %>/session"><input type="date" name="date" required> <button>New session</button></form><% } %>
 <h2>Deck performance <small>(match W-D-L)</small></h2>
 <table><tr><th>Deck</th><th>W-D-L</th></tr><% decks.forEach(([d, r]) => { %><tr><td><%= d %></td><td><%= r.join('-') %></td></tr><% }) %></table>
+<% if (admin) { %><form method="post" action="<%= base %>/admin/league/<%= lg.id %>/delete" data-confirm="Delete this league and all its sessions and results? Players are kept."><button>Delete league</button></form><% } %>