core: implement idempotent transaction submission

[EmeditWeb]

Problem

If a user submits a signed XDR and the API crashes
before confirming, the user might resubmit the same
transaction. No idempotency mechanism exists.

What To Build

1. Add idempotency_key to transaction submission
2. Create idempotency_records table
3. Transaction hash deduplication
4. XDR validation before submission

Files To Touch

• src/modules/blockchain/blockchain.service.ts
• src/modules/blockchain/blockchain.controller.ts
• supabase/migrations/[ts]_idempotency_records.sql

Acceptance Criteria

• Same idempotency key returns same response
• Duplicate XDR hash rejected
• XDR validated before submission
• Records expire after 24 hours

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue