Add a `model_version` field to all AI responses

[kilodesodiq-arch]

Problem Statement. Output schema (schemas/humanitarian.py, schemas/anonymization.py,
schemas/ocr.py) doesn't return the model version. A consumer can't tell if they got the
old gpt-4o-mini or a future gpt-5o.

Technical Context.
app/ai-service/schemas/*.

Expected Outcome. Every response includes model_version, sourced from
settings.openai_model or settings.groq_model. The provider name is already emitted;
this normalises the model disclosure.

Acceptance Criteria.

• A snapshot test pins model_version in expected output.

Files or modules likely to be affected.
app/ai-service/schemas/*.

Difficulty. Easy
Estimated effort. S

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Backlog item #64 from `docs/maintainer-issue-backlog.md.

[Opulencechuks]

I am confident that I can implement this having worked on something similar in the past

[grantfox-oss]

🦊 GrantFox — @Opulencechuks has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #272)
2. Your PR will be reviewed by the ChainForgee maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Opulencechuks's PR #374 was approved and merged by @kilodesodiq-arch.

🏆 @Opulencechuks: You earned 40 FoxPoints for this contribution! Your current tier: Builder (2,574 total points). Track your full progress on GrantFox.

👏 Great work, @Opulencechuks! Keep contributing to ChainForgee.