Stripe whitespace from webhook URL's

[robzolkos]

Copy-pasted webhook URLs (which is probably the most common way users add them) often include accidental trailing whitespace, which type="url" accepts but URI.parse rejects. Because the controller uses create!/update! this results in a silent failure where the form re-renders with cleared input and no error.

This PR normalizes webhook URLs by stripping leading/trailing whitespace before validation. Since trailing whitespace is not meaningful user input and is the only case that slips past browser validation, normalizing at the model boundary avoids unnecessary UX and code complexity compared to rendering form errors.

This shows the before/after (leaving trailing whitespace re-renders cleared form with no explanation and then with this fix in place).

normalization-fix.mp4