Add prerender-django middleware for Django 5+

[Lasalot]

Summary

• Modern rewrite replacing the unmaintained django-seo-js package
• Simplified from a multi-class, multi-backend abstraction to a single clean middleware class
• Zero dependencies — uses urllib.request (stdlib) instead of requests
• Modern Django middleware style (__init__(get_response) + __call__) targeting Django 5+ / Python 3.10+
• Updated bot list with modern crawlers (GPTBot, ClaudeBot, Amazonbot, Perplexity, etc.)

Test plan

• 5 smoke tests covering: browser passthrough, bot interception, static asset bypass, _escaped_fragment_, network error fallback
• Run with pytest inside prerender-django/ (requires Python 3.10+ venv)