[MeshCore P3] M5 - Realtime WS event + recent API

[pskillen]

Parent

Part of #267. Milestone M5.

Goal

Short-term operational view of recent MeshCore packet paths.

Scope

• meshcore.packet_path.observed WS event emitted via transaction.on_commit, best-effort (sampled / coalesced / capped per feeder/channel).
• Redis-backed bounded recent buffer + GET /api/meshcore/path-tracing/recent/ for reconnect hydration.

Acceptance criteria

• No event for rolled-back ingest; no subscriber assumes every observation emits an event.
• Recent endpoint returns a bounded window; documented in OpenAPI.

Out of scope

• UI (M7).