fix: correct README instructions and remove dead code

[golldyck]

Problem

README references a non-existent gemini-3-pro-preview model, tells users to run pip install -r requirements.txt (file does not exist), and uses claude-3.7-sonnet in the curl example which is not in the model registry. Dead code exists in typing_utils.py and an unused constant in defaults.py.

Fix

Corrected the README to match the actual model registry and build system. Removed dead code branches and unused constants.

Changes

• README.md — Replaced gemini-3-pro-preview with gemini-3-flash-preview, changed install command from pip install -r requirements.txt to uv sync, fixed curl example model to one that exists in the registry
• tee_gateway/typing_utils.py — Removed dead code branch for Python < 3.7 (project requires >= 3.12)
• tee_gateway/controllers/defaults.py — Removed unused HTTP_BACKEND_SERVER constant

Testing

• Verified all model names in README exist in tee_gateway/model_registry.py
• Confirmed pyproject.toml is the actual dependency source (no requirements.txt)
• Checked removed code has no references elsewhere in the codebase

Closes #53

[golldyck]

Hi @adambalogh @kylexqian — this PR fixes bugs I found during a full audit of OpenGradient's public repositories. Would appreciate a review when you get a chance. Happy to address any feedback. Thanks!