Implement a bounded-mpsc + background-task Valkey event sink:
- XADD lifecycle events to a capped stream (MAXLEN ~ 10000).
- Fire-and-forget emit(): one mpsc::try_send, count-and-drop on full.
- Arc<AtomicU64> drop counter surfaced in /metrics.
- main.rs selects ValkeyEventSink when RUTSTER_VALKEY_URL is set and
falls back to TracingEventSink otherwise.
- config::valkey_url validates redis:// / rediss://.
- CI services:valkey: block on test and clippy jobs.
Signed-off-by: Aaron D. Lee <himself@adlee.work>
Aaron D. Lee
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