[P2] Image proxy / CDN for cover images

[hazeliscoding]

Problem

Cover images are stored as direct AniList CDN URLs. If AniList changes CDN domains, rotates URLs, or goes down, every image in the app breaks.

Requirements

• Option A: Proxy endpoint (GET /api/images/{anilistMediaId}/cover) that fetches and caches the image
• Option B: Background job that downloads cover images to blob storage (S3/R2) on anime add
• Serve images via a CDN (Cloudflare/CloudFront) with cache headers
• Fallback placeholder image when the cover is unavailable

Acceptance Criteria

• Cover images are served from a URL the app controls
• AniList CDN outages don't break the app's image display
• Images are cached with appropriate headers (long TTL, immutable)
• Missing images show a themed placeholder

References

• Roadmap §6 — Data Integrity & Edge Cases