feat(ws): WebSocket endpoint for real-time price streaming

[stableprogrammer]

Closes #22

Summary

• src/ws/PriceSubscriptionManager.js — core subscription registry: max 5 assets per client, max 100 concurrent connections, notifyPriceUpdates() diffing with 0.1% threshold, heartbeat ping/pong every 30 s, disconnects sockets that miss 3 consecutive pings, increments/decrements ws_connections_current Prometheus gauge (gracefully skipped when prom-client is not installed).

• src/ws/priceWebSocket.js — attaches ws.Server to the existing HTTP server at /ws; starts the heartbeat on attach.

• src/index.js — calls priceWebSocket.attach(server) after http.listen(); stops heartbeat on SIGTERM/SIGINT.

• src/jobs/priceRefresh.js — collects the freshPrices map returned by refreshAllCachedPrices() and passes it to subscriptionManager.notifyPriceUpdates() after each cron cycle.

• src/services/priceOracle.js — refreshAllCachedPrices() now returns { assetKey → { price, source } } so the job can forward diffs to the WS layer.

• test/priceWebSocket.test.js — 7 integration tests (all passing): subscribe confirmation, asset cap at 5, price_update push on >0.1% change, no push below threshold, unsubscribe, invalid JSON error frame, connection count tracking.

Protocol

Client → Server: { "action": "subscribe", "assets": ["XLM", "USDC:GA5Z..."] }
Server → Client: { "type": "subscribed", "assets": ["XLM"] }

Server → Client: { "type": "price_update", "asset": "XLM", "price_usd": 0.1145,
                   "previous_price_usd": 0.112, "change_pct": 2.23,
                   "source": "stellar_dex", "timestamp": "..." }

Server → Client: { "type": "ping" }
Client → Server: { "action": "pong" }

Test plan

• 7 unit/integration tests pass (test/priceWebSocket.test.js)
• Connect to ws://localhost:3000/ws, subscribe to ["XLM"], trigger a price refresh — verify price_update frame arrives
• Verify no frame is sent when price changes < 0.1%
• Let 3 ping cycles elapse without sending pong — verify socket is terminated
• Connect 100 clients — 101st should be rejected with close code 1013