Replace mock with a new client (yfinance)

[BytecodeBrewer]

What changed?

• add yfinance as a new data source
• a client for yfinance
• add and update tests for the new client and the time series service
• the mock client is an obsolete component

Related issue

Closes #33

Tests

• I ran pytest
• Existing tests pass
• I added or updated tests where necessary
• Not needed because this only affects documentation or repository setup

Checklist

• The change is focused
• No secrets or API keys are included
• Documentation was updated if needed
• CI is expected to pass

Notes