CONV-308: Add credits balance endpoint

[menvil]

Summary

• Adds GET /api/v1/credits/balance returning {data: {balance: N, unit: 'credits'}}
• Uses CreditLedger::balance() — no transaction history exposed
• Protected by api.key + api.access middleware

Test plan

• Returns correct balance for Pro user
• Missing API key → 401
• Free user → 403 api_not_available
• composer test passes (584 tests)
• composer lint passes

🤖 Generated with Claude Code

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Summary by cubic

Adds GET /api/v1/credits/balance to let Pro users fetch their current credits balance via API. Addresses CONV-308.

• New Features
  • Response: { data: { balance: N, unit: 'credits' } }.
  • Uses CreditLedger::balance() only; no transaction history.
  • Access control: api.key + api.access required; missing key → 401, Free plan → 403 api_not_available.

^{Written for commit 01eaca3. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/CONV-308-add-credits-balance-endpoint

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