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/session.ejs

@@ -23,10 +23,11 @@
 <p><small>If this session also has match rows, those are used for scoring instead.</small></p><% } %>
 <% if (admin) { %>
 <h2>Import Melee export</h2>
-<p>Upload a <b>matches</b> or <b>standings</b> export from Melee (JSON or CSV), or paste it. A matches export holds one round, so import each round in turn;
-re-importing a round replaces just that round. Decks you've entered are kept. If a session has matches, they are used for scoring.</p>
+<p>Upload the night's Melee exports (JSON or CSV): select every round's <b>matches</b> export and/or the <b>standings</b> export at once, or paste one.
+Re-importing a round replaces just that round; a standings export replaces the imported standings. Decks you've entered are kept.
+If anything in the batch is invalid, nothing is imported. If a session has matches, they are used for scoring.</p>
 <form method="post" enctype="multipart/form-data" action="<%= base %>/admin/session/<%= gs.id %>/import-melee">
-<input type="file" name="file" accept=".csv,.json,.txt"><br>
+<input type="file" name="files" accept=".csv,.json,.txt" multiple><br>
 <textarea name="data" rows="5" style="width:100%;font-family:monospace" placeholder="...or paste here"></textarea><br><button>Import</button></form>
 <h2>Import matches by hand</h2>
 <p>One match per line: <code>round, player1, deck1, player2, deck2, p1 wins, p2 wins, draws</code>. Leave player 2 blank for a bye.