feat: In-app delivery, live WebSocket dashboard, Docker demo stack

[rajatady]

Summary

• Rust in-app delivery provider: Zero-cost delivery that skips HTTP entirely (0μs latency). Lock-free atomic counters for eval/delivery latency instrumentation across all pipeline stages.
• NestJS WebSocket gateway: Standalone ws.Server attached to Fastify's HTTP server at /ws/notifications. Broadcasts delivery events and pipeline metrics (polled every 5s from Rust /health). Redis stream consumer persists to MongoDB and broadcasts live.
• Live frontend dashboard: WebSocket hooks with auto-reconnect, streaming notifications feed with LIVE/OFFLINE indicator, 8-card Pipeline Performance panel (throughput, eval latency avg/max, deliveries, ticks, matches, dropped, connectors).
• Full Docker demo stack: MongoDB, Redis, Rust runtime, NestJS API, Next.js dashboard, seed data, ticker simulator — all wired with health checks and dependencies.

Key decisions

• Used standalone ws.Server with noServer: true instead of NestJS WsAdapter because Fastify intercepts HTTP upgrade requests before the adapter can handle them
• Subscriber cache auto-detects in_app channel when channelPreferences is empty ({}) or null
• Metrics flow: Rust atomics → /health endpoint → NestJS polls every 5s → WebSocket broadcast → React state

Test plan

• cargo check --workspace passes
• NestJS tests pass (247)
• Frontend tests pass (78)
• docker compose up --build — full stack runs
• Dashboard shows live Pipeline Performance metrics via WebSocket
• Notifications page shows in_app deliveries with latency column
• /health endpoint returns deliveries_completed > 0, avg_eval_latency_us > 0
• WebSocket connects and streams delivery + metrics events

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