Ledger REST API Cache friendliness improvements

[citizen-stig]

LedgerDb effectively have 2 types endpoints:

• "live" which should never be cached.
  • /ledger/slots/latest
  • /aggregated-proofs/latest/
• historical, that can be cached forever (for instant finality, or after finalization):
  • /ledger/slots/{slotId}
  • /ledger/txs/{txId}
  • etc..

Currently no Cache-Control headers are set, leaving caching behaviour undefined for clients and intermediaries (CDNs, reverse proxies).

HTTP Header Reference

Historical endpoints

Cache-Control: public, max-age=31536000, immutable

• public — cacheable by CDNs and shared proxies, not just the browser
• max-age=31536000 — 1 year in seconds (practical "forever")
• immutable — tells clients: don't bother revalidating, this URL's content will never change. Only valid for content-addressed URLs (by hash). Do not use on height-based routes.

Live endpoints

Cache-Control: no-store

no-store — strongest option; response must not be written to any cache at any layer. Prefer this over no-cache (which still stores but revalidates) since stale ledger state is actively harmful.