Evented Cache Debug Worker

GET  /r2/{key}        - read directly from R2
GET  /kv/{key}        - read from KV, backfill from R2 on miss
GET  /cache/kv/{key}  - read via Cache API, KV backed by R2 on miss
GET  /cache/r2/{key}  - read via Cache API, R2 only (skips KV)
POST /{key}           - upload body to R2