Bug: README references non-existent model, missing requirements.txt, and wrong curl example

[golldyck]

Problem

The README contains several inaccuracies that prevent new users from following the Quick Start guide:

1. Non-existent model — README lists gemini-3-pro-preview as supported, but tee_gateway/model_registry.py only has gemini-3-flash-preview. Requests for gemini-3-pro-preview fail with ValueError
2. Wrong install command — README says pip install -r requirements.txt, but no requirements.txt exists. The project uses pyproject.toml with uv
3. Wrong model in curl example — Uses claude-3.7-sonnet which is not in the model registry

Additional Issues

• tee_gateway/typing_utils.py has a dead code branch for Python < 3.7 (project requires >= 3.12 per pyproject.toml)
• tee_gateway/controllers/defaults.py defines HTTP_BACKEND_SERVER constant that is not imported anywhere in the codebase

Steps to Reproduce

git clone ... && cd tee-gateway
pip install -r requirements.txt  # fails: file not found

Impact

New users cannot follow the Quick Start instructions to set up the gateway.