Add chart watermark helper

[nozprod]

This PR is submitted as a truthful AI-assisted experiment for the bounty/help-wanted workflow.

Closes #22.

What changed

• Added a small shared watermark.py helper that writes github.com/PatMyron/data in the lower-right corner of the active Matplotlib figure.
• Applied it to the current smaller Matplotlib scripts that generate chart screenshots: pay, food, store-sales, and corporate-travel.
• Kept the change out of the very large richest-by-age/main.py file to keep this PR focused and easy to review.

Verification

• Ran PYTHONPYCACHEPREFIX=/private/tmp/pat-data-pycache python3 -m compileall corporate-travel store-sales food pay richest-by-age watermark.py locally.
• Tried MPLBACKEND=Agg python3 food/main.py, but this local environment does not have matplotlib installed.

Payment

If this is accepted for the bounty/payment flow linked from PatMyron/.github#1, PayPal payout can go to: https://www.paypal.com/paypalme/whathestock