feat: add staging media CDN records

[glitchedmob]

Add the Cloudflare edge configuration for the staging media hostname introduced by sgfdevs/infra-k8s-apps#12.

• Proxy media-staging.sgf.dev to the staging origin
• Cache media responses at the edge for one year
• Respect origin browser TTLs and preserve Cloudflare's query-aware default cache key

[github-actions]

Terraform plan

Artifacts: https://github.com/sgfdevs/infra-dns/actions/runs/31278166403

Show summary

CHANGE	RESOURCE
add (2)	module.sgf_dev.cloudflare_dns_record.core["media_staging"]
	module.sgf_dev.cloudflare_ruleset.cache

Decrypt artifacts:
openssl enc -d -aes-256-cbc -pbkdf2 -in tfplan.enc -out tfplan -pass pass:$OUTPUT_ENCRYPTION_KEY
openssl enc -d -aes-256-cbc -pbkdf2 -in tfplan.txt.enc -out tfplan.txt -pass pass:$OUTPUT_ENCRYPTION_KEY