feat: add Cloudflare Turnstile as alternative captcha provider

[kwitykwity]

Summary

Add support for Cloudflare Turnstile as a privacy-friendly alternative to Google reCAPTCHA for the signup form.

• New setting: CAPTCHA_PROVIDER with options: 'recaptcha', 'turnstile', or 'none'
• Backwards compatible: Existing USE_RECAPTCHA=True configurations continue to work
• Django checks: Added validation warnings when captcha keys are missing

Changes

• Add django-cf-turnstile dependency
• Add CAPTCHA_PROVIDER setting to replace boolean USE_RECAPTCHA
• Add CF_TURNSTILE_SITE_KEY and CF_TURNSTILE_SECRET_KEY settings
• Update signup form to conditionally use either provider
• Add Django system checks for configuration validation
• Add tests for all three captcha modes

Configuration

# Option 1: Cloudflare Turnstile (new)
CAPTCHA_PROVIDER=turnstile
CF_TURNSTILE_SITE_KEY=your_key
CF_TURNSTILE_SECRET_KEY=your_secret

# Option 2: reCAPTCHA (existing)
CAPTCHA_PROVIDER=recaptcha
RECAPTCHA_PUBLIC_KEY=your_key
RECAPTCHA_PRIVATE_KEY=your_secret

# Option 3: No captcha
CAPTCHA_PROVIDER=none

Related Issue(s)

Fixes #1461

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Code has been formatted to avoid unnecessary diffs (ruff format && isort .)
• If the feature is big enough or if there are manual steps needed (deployment changes etc.), write a small writeup in CHANGELOG.md

Testing

DJANGO_SETTINGS_MODULE=settings.ci python manage.py test wger.core.tests.test_registration

All 12 tests pass.

🤖 Generated with Claude Code