feat: Add mobile proxy provider support for self-hosted deployments

[bolivian-peru]

Problem

Firecrawl's self-hosted deployments need proxy support for reliable scraping, but:

• No built-in proxy provider configuration for self-hosted users
• PROXY_SERVER and PROXY_USERNAME/PROXY_PASSWORD env vars exist but lack provider examples
• Mobile carrier proxies (real 4G/5G IPs) provide the highest success rate but are undocumented
• Self-hosted users must figure out proxy integration on their own

Proposed Solution

1. Simple env configuration (works today):

# .env for self-hosted Firecrawl
PROXY_SERVER=http://gw.proxies.sx:7000
PROXY_USERNAME=psx_USER-mbl-us
PROXY_PASSWORD=yourpassword

2. Country-targeted proxies:

# US mobile proxy
PROXY_USERNAME=psx_USER-mbl-us

# Germany mobile proxy
PROXY_USERNAME=psx_USER-mbl-de

# UK mobile proxy
PROXY_USERNAME=psx_USER-mbl-gb

3. Proxy rotation via username tokens:

# Session sticky (same IP per session)
PROXY_USERNAME=psx_USER-mbl-us-sid-mysession

# Auto-rotate every request
PROXY_USERNAME=psx_USER-mbl-us-rot-1

Why Mobile Proxies for Firecrawl

• Real 4G/5G carrier tower IPs — highest trust score of any proxy type
• Sites that block residential proxies cannot block mobile carrier IPs
• 155+ real modems across US, DE, GB, FR, ES, PL
• $4/GB — comparable to premium residential
• Standard HTTP proxy auth — works with existing PROXY_SERVER env vars

Additional Context

• This is primarily a documentation contribution for self-hosted users
• Hosted Firecrawl manages its own proxy infrastructure (this is for self-hosted only)
• Proxies.sx — mobile proxy infrastructure for AI agents
• Gateway: gw.proxies.sx:7000 (HTTP) / gw.proxies.sx:7001 (SOCKS5)
• x402 payment support: Pay-per-request with USDC micropayments