Add the yaak.run playground API (#681)

crates-server/yaak-playground/README.md

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+# yaak-playground
+
+The API behind [yaak.run](https://yaak.run), for trying Yaak without bringing your own.
+
+- Users and todos are fixed sample data.
+- Posts can be created, changed, and deleted. Each client IP gets its own copy, which goes back
+  to the sample data `YAAK_PLAYGROUND_RESET_AFTER_SECS` after its first write.
+- `POST /auth/token` returns a bearer token for `GET /auth/me`. `POST /auth/session` sets a
+  session cookie for `GET /auth/session`. Any username with the password `yaak` logs in.
+- `GET /` lists every endpoint and `GET /openapi.json` describes them.
+
+Nothing is written to disk, so a restart resets everything.
+
+```sh
+cargo run -p yaak-playground   # http://127.0.0.1:9228
+```
+
+Run `yaak-playground --help` for the limits. Behind a load balancer that sets
+`X-Forwarded-For`, set `YAAK_PLAYGROUND_TRUST_FORWARDED_FOR=true`.
+
+The image is `Dockerfile.playground` at the repo root.