feat: add Cheaper Inference as AI provider

[aiapienthusiast]

📋 Description

This PR adds Cheaper Inference as an LLM provider in the backend.
It uses the same pattern as #147 (Requesty).

Cheaper Inference is an OpenAI-compatible LLM gateway.
One API key gives access to models from several labs.

Usage:

LLM_PROVIDER=cheaperinference
CHEAPER_INFERENCE_API_KEY=ci_live_...
CHEAPER_INFERENCE_MODEL=gpt-5.4-mini

Embeddings use the OpenAI settings, like the MiniMax provider.

Details

Files:

• backend/src/utils/llm/models/cheaperinference.ts: new provider. It uses ChatOpenAI with base URL https://api.cheaperinference.com/v1.
• backend/src/utils/llm/models/index.ts: add the cheaperinference case.
• backend/src/config/env.ts: read CHEAPER_INFERENCE_API_KEY, CHEAPER_INFERENCE_MODEL, CHEAPER_INFERENCE_BASE_URL.
• backend/src/utils/llm/models/__tests__/cheaperinference.test.ts: new unit test with a mocked @langchain/openai.
• .env.example, README.md: document the provider.

Tests:

• npx vitest run: 7 files passed, 1 skipped (41 tests passed, 3 skipped).
• npm run build: pass.
• The unit test uses mocks.
• Live check: gpt-5.4-mini returned a reply through makeModels() with LLM_PROVIDER=cheaperinference.

🔄 Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🎨 Style/UI changes
• ♻️ Code refactoring
• ⚡ Performance improvements
• 🧪 Test updates
• 🔧 Build/CI changes

🧪 Testing

• I have tested this change locally
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

📱 Screenshots (if applicable)

Not applicable. No UI change.

🔍 Code Review Checklist

• Code follows the project's coding standards
• Self-review of the code has been performed
• Code is properly commented, particularly in hard-to-understand areas
• Changes generate no new warnings
• Any dependent changes have been merged and published

📚 Related Issues

None.

🚀 Deployment Notes

No change for current users.
The provider is active only with LLM_PROVIDER=cheaperinference.

📋 Additional Context

Keys: https://cheaperinference.com/signup