Investigate Cloudflare cache rules not applying to R2 custom domain (litmus-7zy0)

Cache rules created under the correct zone for screenshots.litmus.edger.dev but cf-cache-status remains DYNAMIC on all requests. Images and manifest serve correctly, just not being cached at the edge.

Context

• Two cache rules active: immutable images (1yr TTL) and manifest short TTL (1min)
• Both rules set to ‘Eligible for cache’ with ‘Ignore cache-control header and use this TTL’
• R2 bucket serves via custom domain, CORS working
• Rules confirmed on the correct zone

To investigate

• Check if R2 custom domain responses bypass cache by default
• Check Cloudflare docs for R2 + cache rules interaction
• Try a Cache Everything page rule as alternative — not needed
• Check if the zone plan level affects R2 caching behavior — no, affects all plans

Summary of Changes

Root cause: R2 does not set Cache-Control headers by default. Cloudflare cache rules do not reliably override this for R2 custom domains — a known limitation where R2 responses bypass the normal cache layer.

Fix: Updated screenshots-sync mise task to set Cache-Control headers via rclone --header-upload:

• Images: public, max-age=31536000, immutable (1 year)
• Manifest: public, max-age=60 (1 minute)

Added screenshots-sync_ignore-checksum task for one-time force re-upload.

Verified: cf-cache-status: HIT confirmed on both manifest and images.