[Backend] Replace duplicated isValidStellarAddress in remittanceService with shared util

[ogazboiz]

Telegram: https://t.me/+DOylgFv1jyJlNzM0

Why this matters

backend/src/services/remittanceService.ts (around line 42) defines a private isValidStellarAddress that is a byte-for-byte duplicate of the canonical one already exported from backend/src/utils/stellar.ts. Duplicated validators drift over time (the util copy already disagrees on its regex), so a single source of truth prevents bugs and reduces maintenance.

Acceptance criteria

• Import isValidStellarAddress from ../utils/stellar.js in remittanceService.ts
• Delete the local copy
• Run the existing remittanceService.test.ts suite to confirm no regression

Files to touch

• backend/src/services/remittanceService.ts
• backend/src/utils/stellar.ts (import only)

Out of scope

• Sharing the validator across the frontend package (different package boundary)

[TheShnider]

@TheShnider has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi. I have reviewed this issue and would like to work on it.

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[wandooadzer-cmyk]

@wandooadzer-cmyk has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can I work on this task?

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