CRITICAL: Cooldown was global across ALL symbols, causing missed opportunities
Example: ETH trade at 10:00 blocked SOL trade at 10:04 (5min cooldown)
Changes:
- Added getLastTradeTimeForSymbol() function to query last trade per symbol
- Updated check-risk endpoint to use symbol-specific cooldown
- Each coin (SOL/ETH/BTC) now has independent cooldown timer
- Cooldown message shows symbol: 'Must wait X min before next SOL-PERP trade'
Result: Can trade ETH and SOL simultaneously without interference
Example: ETH LONG at 10:00, SOL SHORT at 10:01 = both allowed
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