[backend] Implement getBetsByAddress() service

[Ehonrie]

Implement getBetsByAddress() in backend/src/services/bet.service.ts.

What to implement:

• Query Bet where bettor == address
• Support optional filters: status and marketId
• Order by placedAt descending

Acceptance Criteria:

• Returns all bets for a known address
• Returns empty array for unknown address
• Status filter returns only matching bets
• Integration test covers all filter combinations

[martoniel]

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

Good day Big Dev! Pls I would love to work on this issue carefully and make PR today

ℹ️ Repo Maintainers: To accept this application, review their application or assign @martoniel to this issue.

[starkiVector-090]

@starkiVector-090 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi team, I would love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @starkiVector-090 to this issue.

[drips-wave]

Congratulations, @starkiVector-090! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @starkiVector-090: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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