Fix toggle-favourite HTTPException handling and add DB helper + tests. #issue 1179

[Cairo09]

Motivation

• Prevent the toggle_favourite() route from converting intended HTTPException responses (e.g. 404) into generic 500 errors.
• Avoid an expensive full-table fetch after toggling the favourite flag by retrieving only the updated isFavourite value.
• Provide API-level tests to assert correct 200 and 404 behaviors for the toggle endpoint.

Description

• Updated backend/app/routes/images.py to explicitly re-raise HTTPException and handle unexpected errors with a generic except Exception returning 500.
• Replaced the full-table lookup in toggle_favourite() with a call to the new helper db_get_image_favourite_status(image_id) and used that value in the response.
• Added db_get_image_favourite_status() and switched db_toggle_image_favourite_status() to use the shared _connect() helper in backend/app/database/images.py.
• Added focused API tests in backend/tests/test_images.py that mock DB calls to cover a successful toggle and the unknown-image 404 case.

Testing

• Ran python -m py_compile backend/app/routes/images.py backend/app/database/images.py backend/tests/test_images.py which succeeded.
• Executed pytest backend/tests/test_images.py in this environment and it failed during test bootstrap due to an external system library import error (cv2 -> libGL.so.1 missing), preventing full test execution.
• The new tests are unit-level and mock DB functions, and they should pass in a normal CI/test environment where the test bootstrap imports resolve successfully.

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