fix: resolve competition place deduction crashes in purchasePlaces

[SiRipo92]

Closes #6

Problem

purchasePlaces() crashed with IndexError 500 when competition or club
names from the form did not match the database. [0] was called on an
empty list with no error handling, making the function unreachable in
error scenarios.

Root Cause

Two instances of [0] list indexing assumed a match would always be found:
competition = [c for c in competitions if c['name'] == ...][0]
club = [c for c in clubs if c['name'] == ...][0]

Fix

• Replaced [0] with next() and None default for both competition and club
• Added guard clause: if not competition or not club flash error and
  redirect to welcome.html keeping the user logged in
• PEP8: placesRequired → places_required, added spaces around operator

Tests

• HAPPY PATH: valid booking deducts places from competition
• HAPPY PATH: updated count reflected immediately in memory
• SAD PATH: unknown competition returns error without crashing
• SAD PATH: unknown club returns error without crashing
• EDGE CASE/ CHARACTERISATION: numberOfPlaces type consistency after deduction