feat: add WebSocket endpoint for real-time price updates

[olathedev]

Summary

Closes #80

• ws://host/api/ws/prices — WebSocket endpoint on the same port as the REST API (no extra server needed)
• Subscribe/unsubscribe — clients send { type: "subscribe", assets: ["XLM","BTC"] } or ["*"] for all assets
• Push-only updates — prices are only broadcast when the value changes; unchanged prices are suppressed
• Instant cache — subscribers receive last known price immediately on connect, before the next poll cycle
• Heartbeat — server pings every 30 s to detect and clean up stale connections
• Configurable polling — WS_PRICE_UPDATE_INTERVAL_MS (default 30 s), falls back from ORACLE_API_URL to CoinGecko public API

Message protocol

// Client → Server
{ "type": "subscribe",   "assets": ["XLM", "BTC"] }  // or ["*"] for all
{ "type": "unsubscribe", "assets": ["XLM"] }
{ "type": "ping" }

// Server → Client
{ "type": "subscribed",    "assets": ["XLM", "BTC"] }
{ "type": "price_update",  "asset": "XLM", "price": 0.12, "timestamp": 1711234567 }
{ "type": "unsubscribed",  "assets": ["XLM"] }
{ "type": "pong" }
{ "type": "error",         "message": "..." }

New env vars

Variable	Default	Description
ORACLE_API_URL	(empty)	Oracle service base URL; falls back to CoinGecko
WS_PRICE_UPDATE_INTERVAL_MS	30000	Price poll interval
WS_HEARTBEAT_INTERVAL_MS	30000	Heartbeat ping interval

Test plan

• Connect a WebSocket client to ws://localhost:3000/api/ws/prices
• Send { "type": "subscribe", "assets": ["XLM"] } — should receive subscribed ack
• Wait for next poll cycle — should receive price_update for XLM
• Send { "type": "ping" } — should receive pong
• Send { "type": "unsubscribe", "assets": ["XLM"] } — no further XLM updates

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