feat(payments): implement crypto deposit address generation for Stell…

[Wisdom2788]

Closes #10

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Description

I implemented a dynamic crypto deposit address generation for Stellar, Bitcoin, and Ethereum blockchains using HD wallet derivation.

Changes

• Added DepositAddress Prisma schema linked to PaymentIntent
• Implemented SLIP-0010 Ed25519 derivation for Stellar (BIP-44 path m/44'/148'/index')
• Implemented BIP-44 derivation for Ethereum via ethers v6 HDNodeWallet (m/44'/60'/0'/0/index)
• Implemented BIP-44 derivation for Bitcoin via bitcoinjs-lib + bip32 (m/44'/0'/0'/0/index)
• Added PrismaModule (global, degraded mode until DATABASE_URL is set)
• Added ConfigModule for .env loading
• Idempotent address generation — same intent + chain always returns same address
• All endpoints protected by JWT guard

Endpoints

• POST /payments/intents/:intentId/deposit-address — generate address
• GET /payments/intents/:intentId/deposit-address — list all addresses for intent
• GET /payments/intents/:intentId/deposit-address/:chain — get by chain

Testing

Tested locally — all three chains return valid addresses with correct derivation indices.

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