Use NATS JetStream KV store watchers with UpdatesOnly and TTL-based purge for live distributed configuration

Steps

1. Create a KV bucket with a TTL: js.CreateKeyValue(jetstream.KeyValueConfig{Bucket: "config", TTL: 24 * time.Hour}) — entries not updated within the TTL are automatically purged
2. Write configuration values: kv.Put(ctx, "feature.dark-mode", []byte("enabled"))
3. Subscribe to changes using a watcher with UpdatesOnly option to receive only deltas after the initial snapshot: watcher, _ := kv.WatchAll(ctx, jetstream.UpdatesOnly()); this prevents replaying the entire history on reconnect
4. Consume watch entries: for entry := range watcher.Updates() { if entry == nil { break } /* handle entry */ } — a nil entry signals the end of the initial values delivery (even with UpdatesOnly, a single nil is sent to mark readiness)
5. Handle Delete and Purge operations in the watcher: entry.Operation() returns KeyValuePut, KeyValueDelete, or KeyValuePurge; remove local cache entries on delete/purge operations
6. Perform a TTL-scoped purge for key retirement: kv.Purge(ctx, "feature.old-flag", jetstream.PurgeTTL(time.Hour)) — the purge marker remains visible for 1 hour before disappearing