Add StoreDeployedConfigIfNewerAsync to SiteStorageService — an atomic
conditional upsert using SQLite's ON CONFLICT … WHERE clause so the
standby node only overwrites a deployed_configurations row when the
incoming deployed_at is strictly newer. The active-node path
(StoreDeployedConfigAsync) stays unguarded. Four deterministic tests
cover: no-row insert, older-overwrites, newer-is-noop, equal-is-noop;
all seed rows with explicit DateTimeOffset values via direct SQL to
avoid wall-clock timing dependencies.
Joseph Doherty