Add a test seeder behind the unsafe_test_mode feature

For the end-to-end tests: with the feature, /test/seeder answers the calls the
SDK's live tests make to a seeder API (POST /seed with SingleUserScene or
TestVectorUserScene, DELETE /seed/<playId>), behind HTTP basic auth from
TEST_SEEDER_USERNAME and TEST_SEEDER_PASSWORD, and 404 without them.

It creates accounts without any checks, so no release build has the feature.
CI builds and tests it with sqlite.

Cargo.lock

@@ -6270,6 +6270,7 @@ dependencies = [
  "rustls",
  "semver",
  "serde",
+ "serde_cbor_2",
  "serde_json",
  "subtle",
  "svg-hush",