feat: publish signals to Redis Pub/Sub

[yuriy-arabskyy]

Context

Currently, signals are only written to QuestDB metrics. The admin portal polls QuestDB every 10s to display them. For real-time display, we need signals published to Redis Pub/Sub so they can be streamed to the frontend instantly.

Requirements

• When a strategy generates a signal, publish it to Redis channel bot:{bot_id}:signals
• Message format: JSON with timestamp, symbol, exchange, market_type, signal value, reference_price, price, take, stop, group_name, comment, strategy_name
• Publishing should be fire-and-forget (non-blocking) — don't let Redis Pub/Sub failures affect trading
• Use existing Redis connection from bot config (REDIS_URL in bot-config secret)

Notes

• Keep existing QuestDB metric writes — Redis Pub/Sub is additive
• Consider batching if signal rate is very high (unlikely for most strategies)

Acceptance Criteria

• Signals published to Redis Pub/Sub channel in real-time
• Message format matches what the frontend expects (same fields as /bots/{id}/signals response)
• No impact on trading latency if Redis is slow/unavailable
• Works with existing bot-config Redis URL