Add custom prompt functionality

[Aatif-Dawawala]

Description

These changes implement passing custom prompts to models as shown in the README.

Related Issues

Vision Service: Improve caption prompt and allow submitting a custom prompt with the request data

Acceptance Criteria

• New features or enhancements are fully implemented and do not break existing functionality, so that they can be released at any time without requiring additional work
• [N/A] Automated unit and/or acceptance tests are included to ensure that changes work as expected and to reduce repetitive manual work
• [N/A] Documentation has been / will be updated, especially as it relates to new configuration options or potentially disruptive changes

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[derneuere]

Great to have you back on this project! 🚀

[lastzero]

What if prompt == '' (empty)?

[lastzero]

Also, please check / confirm whether the following environment variables can still be used to set a default prompt for the corresponding endpoints:

• OLLAMA_NSFW_PROMPT
• OLLAMA_LABELS_PROMPT
• OLLAMA_CAPTION_PROMPT

If they are set to an empty string or "default", the built-in default should be used. This default must never be "default" or empty.

[Aatif-Dawawala]

What if prompt == '' (empty)?

If the user doesn't supply a prompt or the prompt is an empty string, the default prompt will be used.

[lastzero]

Note that I changed the caption default prompt to work well with the qwen2.5vl model.

[lastzero]

I manually applied your changes for the caption prompt so that we can start testing it while you finalize the pull request :)