feat: add get_pool_stats with optional odds calculation (#418)

[ayomideadeniran]

• Add PoolStats struct with comprehensive pool statistics
• Implement get_pool_stats(pool_id, include_odds) function
• Optional odds calculation to optimize performance for large pools
• Odds represented as fixed-point numbers with 4 decimal precision (e.g., 7500 = 0.75)
• When include_odds=false, returns None to skip expensive calculation
• When pool has no stakes, odds is None even if include_odds=true

Implementation details:

• Avoids floating point arithmetic by using integer math with 10_000 multiplier
• Checks overflow in odds calculation
• Iterates through all outcomes only when include_odds=true and total_stake>0

Tests added:

• test_get_pool_stats_without_odds: Verify basic stats without odds calculation
• test_get_pool_stats_with_odds: Verify odds are calculated correctly with 4 decimals
• test_get_pool_stats_empty_pool: Handle pools with no predictions
• test_get_pool_stats_resolved_pool: Verify stats work for resolved pools

Fixes #418

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