[contracts] Add BetPlaced event emission in place_bet()

[Ehonrie]

Add BetPlaced event to place_bet() in contracts/market/src/lib.rs.

Event must include: bet_id, market_id, bettor, side, amount, placed_at.

Acceptance Criteria:

• Event emitted on every successful place_bet() call
• All required fields present
• Unit test asserts event values

[adefemiesther1-debug]

@adefemiesther1-debug has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i will love to work on this issue

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[drips-wave]

Congratulations, @adefemiesther1-debug! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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