[storage.cassandra] accepts unknown keys silently #540

CassandraStorageConfig (crates/storage-cassandra/src/config.rs:11) is the one configuration struct without #[serde(deny_unknown_fields)]. Every other section, including PostgresStorageConfig (crates/storage-postgres/src/config.rs:10), refuses to start on an unknown key with an "unknown field" error. In [storage.cassandra] a misspelled key (replication_factr = 3) or a key from another backend is ignored silently and the default applies.

The configuration reference (#537) names the exception. The fix is the one attribute, plus a test that a misspelled key fails deserialization, and a check of the deployment fixtures and the init template for any key the struct does not have, since those would start failing. It changes what the server accepts, which is why it is not part of the documentation change.

Found while checking every configuration key against the code that reads it, for #494.