Implement apply_interest() in bank.py

[hummusonrails]

Summary

Implements Account.apply_interest() in bank.py.

The method now:

• rejects negative interest rates without changing account state
• calculates interest as a percentage of the current balance
• adds the computed interest to the account balance
• records the interest transaction as (\"Interest\", interest)
• returns True when interest is applied successfully

Validation

python3 - <<'PY'
from bank import Account

acct = Account('A1', 'Ada', 100.0)
assert acct.apply_interest(5) is True
assert acct.balance == 105.0
assert acct.transactions == [('Interest', 5.0)]
assert acct.apply_interest(0) is True
assert acct.balance == 105.0
assert acct.transactions[-1] == ('Interest', 0.0)
assert acct.apply_interest(-1) is False
assert acct.balance == 105.0
print('apply_interest checks passed')
PY

Result: apply_interest checks passed

Closes #35