Issue #4: Implement Send Transaction Endpoint

[Chinaza007]

Labels: feature, backend, stellar, intermediate

Description:
Create REST API endpoint to submit transactions to Stellar network.
Acceptance Criteria:

POST /api/v1/transactions endpoint
Validates recipient address (valid Stellar public key)
Validates amount (positive, sufficient balance)
Builds and submits Stellar transaction
Stores transaction in database
Returns transaction hash and status
Handles Stellar network errors gracefully
Rate limiting (10 transactions per minute per user)

Request Body:
json{
"recipientPublicKey": "GDJK...",
"amount": "50.00",
"asset": "USDC",
"memo": "For groceries"
}
Response:
json{
"id": "uuid",
"hash": "stellar-tx-hash",
"status": "pending",
"createdAt": "2026-02-13T14:30:00Z"
}
Estimated Time: 6-8 hours