fix: handle unknown email gracefully in showSummary

[SiRipo92]

Closes #2

Problem

When a user submitted an email not found in clubs.json, the app crashed
with an IndexError 500. The [0] index was called on an empty list with
no error handling in place.

Root Cause

list comprehension + [0] assumed a match would always be found:
club = [club for club in clubs if club['email'] == request.form['email']][0]

Fix

Replaced [0] list indexing with next() and a None default, allowing the
absent case to be handled explicitly rather than crashing:
club = next((club for club in clubs if club['email'] == request.form['email']), None)

Added if not club guard to flash an error message and redirect back to
index.html without crashing.

Updated index.html and welcome.html to display flash messages.

Tests

• SAD PATH: unknown email returns 200 + error message
• HAPPY PATH: valid email loads welcome/summary page
• EDGE CASE: empty email submission handled gracefully