Added project thumbnail endpoint

[A-Artemis]

Summary

This PR adds the GET project thumbnail endpoint. It also links the thumbnail's ID to the project for consistency.

How to test

http://geti-tune.localhost/api/projects/9d6af8e8-6017-4ebe-9126-33aae739c5fa/thumbnail

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have ran e2e tests and there is no issues.
• I have added the description of my changes into CHANGELOG in my target branch (e.g., CHANGELOG in develop).​
• I have updated the documentation in my target branch accordingly (e.g., documentation in develop).
• I have linked related issues.

License

• I submit my code changes under the same Apache License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2025 Intel Corporation
# SPDX-License-Identifier: Apache-2.0

[github-actions]

📊 Test coverage report

Metric	Coverage
Lines	25.8%
Functions	29.8%
Branches	84.7%
Statements	25.8%

[github-actions]

Docker Image Sizes

Image	Size
geti-tune-pr-4747	1.2G
geti-tune-sha-cac3b62	1.2G

[Copilot]

Pull Request Overview

This PR adds a GET endpoint for project thumbnails and links thumbnail IDs to projects. The implementation automatically selects and associates a thumbnail with the project if none is set.

• Adds thumbnail_id field to both Project schema and database model
• Implements GET /api/projects/{project_id}/thumbnail endpoint that returns thumbnail images
• Adds logic to automatically select and link thumbnails from dataset items when no explicit thumbnail is set

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
backend/app/schemas/project.py	Adds optional thumbnail_id field to Project schema
backend/app/db/schema.py	Adds thumbnail_id column to ProjectDB table
backend/app/services/mappers/project_mapper.py	Updates mapper to handle thumbnail_id field conversion
backend/app/services/project_service.py	Adds thumbnail path resolution logic and automatic thumbnail selection
backend/app/api/endpoints/projects.py	Implements GET thumbnail endpoint
backend/app/api/dependencies.py	Updates ProjectService dependency to accept data_dir parameter
backend/app/alembic/versions/95b8e3d1e0a7_schema.py	Database migration to add thumbnail_id column
backend/tests/unit/services/mappers/test_project_mapper.py	Adds test coverage for thumbnail_id mapping
backend/tests/unit/endpoints/test_projects.py	Adds unit tests for thumbnail endpoint
backend/tests/integration/services/test_project_service.py	Adds integration tests for thumbnail functionality
backend/tests/integration/services/test_dataset_service.py	Updates fixture naming for consistency
backend/tests/integration/alembic/test_initial_schema_migration.py	Updates expected migration version

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