fix: Remove fake HAPPY_CODER_API_KEY, use actual Happy Coder env vars

Happy Coder does not require an API key. It uses these environment variables:
- HAPPY_SERVER_URL (optional custom server)
- HAPPY_WEBAPP_URL (optional custom webapp)
- HAPPY_HOME_DIR (data directory)
- HAPPY_EXPERIMENTAL (enable experimental features)

Changes:
- Remove HAPPY_CODER_API_KEY from all files
- Add proper Happy Coder environment variables to StatefulSet
- Add Happy Coder config options to ConfigMap
- Update README with correct Happy Coder configuration
- Update Makefile and docker-compose examples
- Update secrets-example.yaml

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

k8s/secrets-example.yaml

@@ -11,8 +11,5 @@ stringData:
   # GitHub Personal Access Token (for private repos)
   github-token: "ghp_your_token_here"
 
-  # Happy Coder API Key
-  happy-coder-api-key: "your_happy_coder_api_key"
-
   # VNC Password (optional, for secure VNC access)
   vnc-password: "your_vnc_password"