perf: avoid full table scan in toggle-favourite endpoint

[ajithhraj]

What happened?

In backend/app/routes/images.py, the /toggle-favourite endpoint fetches all images from the database just to retrieve the updated favourite status of a single image:

image = next(
    (img for img in db_get_all_images() if img["id"] == image_id), None
)

This is a full table scan on every toggle action. If a user has thousands of images, this is extremely slow and wasteful.

Why this matters

• Poor performance at scale
• Unnecessary database load on every favourite toggle
• db_get_all_images() is an expensive query with JOINs

Proposed fix

Add a db_get_image_by_id(image_id) function to database/images.py and use it instead of scanning all images.

Expected behavior

The endpoint should fetch only the single toggled image using its ID.

I'd like to fix this if maintainers agree.

[github-actions]

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