Implement durable sponsored Stellar account funding workflow

[afeezorobsco-cyber]

#close #91

Summary

This PR implements a durable sponsored Stellar account funding workflow with idempotent transaction handling, persistent funding state, reconciliation, and secure funding execution.

Changes

Funding Policy

• Added configurable funding policy for sponsored account creation.
• Supports configurable funding amount, funding source, eligibility rules, and operational limits.

Funding Job

• Added asynchronous funding job using the existing outbox/job infrastructure.
• Processes newly provisioned Stellar accounts safely and idempotently.

Transaction State

• Persisted funding lifecycle information including:
  • idempotency key
  • transaction status
  • transaction hash
  • ledger
  • retry metadata
  • provider errors
  • final state

Idempotency

• Prevents duplicate funding during retries.
• Reconciles transactions after timeout-before-confirmation scenarios.
• Recovers existing transaction state before retrying.

Already Funded Detection

• Detects accounts that already meet the funding requirements.
• Skips unnecessary funding transactions.

Failure Handling

• Handles:
  • provider outages
  • insufficient funding balance
  • transient failures
  • delayed confirmations
  • manual review scenarios

Security

• Funding secrets are never logged.
• Funding secrets are never persisted.
• Sensitive configuration remains protected.

Testing

Added mocked Horizon tests covering:

• duplicate execution
• idempotency
• timeout-after-submit
• reconciliation
• already-funded detection
• provider failures
• insufficient balance
• confirmation handling
• funding policy enforcement
• secret redaction

Verification

• ✅ Tests pass
• ✅ Funding is idempotent
• ✅ Transaction hash and ledger are persisted
• ✅ Funding secrets remain protected
• ✅ Mocked Horizon integration passes

[afeezorobsco-cyber]

@3m1n3nc3 pls review and merge