feat(video): add RedGifs video support with proxy

[mtsz-pl]

Add support for RedGifs videos embedded in Reddit posts. Videos are proxied through redlib for privacy, similar to v.redd.it handling.

Features:

• Detect RedGifs posts and proxy videos through /redgifs/ endpoint
• Two-step flow: video ID lookup via API, then proxy video file
• Token caching with 24h expiry for RedGifs API authentication
• Prefer HD quality, fallback to SD automatically
• Lazy loading with preload="metadata" to save bandwidth

Security:

• Strict domain validation (only legitimate redgifs.com domains)
• File extension validation (only .mp4 files)
• Query parameter stripping from video IDs
• Pattern matching for all versioned CDN subdomains (v1, v2, etc.)

Implementation:

• New redgifs module with API integration
• Reuses existing proxy infrastructure
• Domain validation helper for consistent security checks

Fixes #20