Implement apply_interest in bank.py

[hummusonrails]

Summary

• Implements Account.apply_interest() with numeric non-negative rate validation
• Updates account balance by applying the given rate
• Records the applied interest amount in the transaction history
• Adds focused regression tests for normal, zero, and invalid-rate cases

Verification

• python3 -m py_compile bank.py test_apply_interest.py
• python3 -c "from bank import Account; a=Account('1','Ada',100.0); assert a.apply_interest(0.05); assert a.balance==105.0; assert a.transactions[-1]==('Interest Applied',5.0); assert not a.apply_interest(-0.01)"

/claim #35