fix: Add missing env_file to UI service in docker-compose

[frederikb96]

Description

This PR fixes inconsistent environment variable handling in docker-compose.yml that breaks documented workflows.

Current Issues:

1. UI .env file ignored - README instructs users to create ui/.env, but docker-compose doesn't load it (only works via Makefile)
2. Confusing API_KEY variable - Listed in docker-compose and config.json but unused (codebase expects OPENAI_API_KEY)

What This Fixes:

• Adds env_file: - ui/.env to UI service → Makes documented workflow (cp ui/.env.example ui/.env) actually work
• Removes unused API_KEY env variable from API service → Reduces confusion
• Standardizes config.json to use env:OPENAI_API_KEY → Aligns with codebase expectations
• Documents Makefile precedence behavior → Users understand when .env values are overridden

Environment Variable Precedence (after fix):

• Makefile env vars > .env file > empty
• Both make up and docker compose up now work correctly

Type of change

• [x] 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)
• [ ] Refactor (does not change functionality, e.g. code style improvements, linting)
• [ ] Documentation update

How Has This Been Tested?

Manual testing with both workflows:

• make up - Still works (Makefile variables take precedence)

• docker compose up - Now works with ui/.env fallback (previously broken)

• [ ] Unit Test

• [x] Manual Testing (verified both Makefile and direct docker-compose workflows)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas (N/A - YAML config changes)
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works (N/A - infrastructure config)
• [ ] New and existing unit tests pass locally with my changes (N/A - no test changes)
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings

[frederikb96]

@parshvadaftari I rebased again on main, ready to merge? :slightly_smiling_face: