Update tianji-install.sh: Add OPENAI_API_KEY to .env

[michelroegl-brunner]

🛠️ Note:
We are meticulous about merging code into the main branch, so please understand that pull requests not meeting the project's standards may be rejected. It's never personal!
🎮 Note for game-related scripts: These have a lower likelihood of being merged.

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✍️ Description

Fixes a issue with tianji wich needs a OPENAI_API_KEY enviroment variable. It can be empty.

Befor you update please set OPENAI_API_KEY="" in /opt/tianji/src/server/.env. Otherwise the service failes to start.

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Please remove unneeded lines!

• Related Issue: Tianji Installation - Cannot Connect #1479
• Docker - OPENAI_API_KEY environment variable is missing or empty msgbyte/tianji#139

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🛠️ Type of Change

Please check the relevant options:

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change unexpectedly)
• New script (a fully functional and thoroughly tested script or set of scripts)

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✅ Prerequisites

The following steps must be completed for the pull request to be considered:

• Self-review performed (I have reviewed my code to ensure it follows established patterns and conventions.)
• Testing performed (I have thoroughly tested my changes and verified expected functionality.)
• Documentation updated (I have updated any relevant documentation)

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📋 Additional Information (optional)

Provide any extra context or screenshots about the feature or fix here.