Update Azure API key env var name and switch to branch-based workflow

[skytin1004]

I noticed that the Azure AI Service API key was not properly configured, which caused Co-op Translator to run in markdown-only mode.

To address this, I updated the environment variable from AZURE_SUBSCRIPTION_KEY to the correct AZURE_AI_SERVICE_API_KEY.
Could you please check whether the corresponding GitHub secret (AZURE_AI_SERVICE_API_KEY) is properly configured in this repository?

AZURE_AI_SERVICE_API_KEY: ${{ secrets.AZURE_AI_SERVICE_API_KEY }}
AZURE_AI_SERVICE_ENDPOINT: ${{ secrets.AZURE_AI_SERVICE_ENDPOINT }}
AZURE_OPENAI_API_KEY: ${{ secrets.AZURE_OPENAI_API_KEY }}
AZURE_OPENAI_ENDPOINT: ${{ secrets.AZURE_OPENAI_ENDPOINT }}
AZURE_OPENAI_MODEL_NAME: ${{ secrets.AZURE_OPENAI_MODEL_NAME }}
AZURE_OPENAI_CHAT_DEPLOYMENT_NAME: ${{ secrets.AZURE_OPENAI_CHAT_DEPLOYMENT_NAME }}
AZURE_OPENAI_API_VERSION: ${{ secrets.AZURE_OPENAI_API_VERSION }}
OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
OPENAI_ORG_ID: ${{ secrets.OPENAI_ORG_ID }}
OPENAI_CHAT_MODEL_ID: ${{ secrets.OPENAI_CHAT_MODEL_ID }}
OPENAI_BASE_URL: ${{ secrets.OPENAI_BASE_URL }}

In addition, I changed the workflow trigger to a branch-based trigger instead of PR-based.
This avoids CLA license check failures when PRs are created by the Co-op Translator bot.

[github-actions]

👋 Thanks for contributing @skytin1004! We will review the pull request and get back to you soon.

[leestott]

@skytin1004 thanks for spotting this!