Fix setup docs and move API keys to environment variables

[Esc1200]

Summary

• Add a requirements.txt so the documented install command works
• Add .env.example and load Gemini/Pexels API keys from environment variables
• Update README setup/run instructions to match the current edge-tts based code path and avatar filename

Testing

• uv venv .venv
• uv pip install -r requirements.txt
• python -m py_compile main.py modules/*.py
• Verified AudioEngine import and edge-tts audio generation
• Verified missing GEMINI_API_KEY and PEXELS_API_KEY now fail with clear RuntimeError messages
• Rendered a short local test video using the included avatar clip as a stand-in visual