Switch to single-key storage or D1

[berrysauce]

Currently, junk.boats uses multi-key storage with suffix IDs for every stored email:

lazy.koala291@junk.boats-19nw729
lazy.koala291@junk.boats-82hs829
lazy.koala291@junk.boats-19ma637

Due to the high costs for listing a KV storage's keys by prefix, junk.boats should use single-key storage instead:

lazy.koala291@junk.boats
- 19nw729
- 82hs829
- 19ma637

Or alternatively, switch to the new relational NoSQL D1 database by Cloudflare.