BUG: The Case of the Missing Pointer

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

**Where** : `main/backend/app/database/images.py`

**What happened ?** : `db_get_images_by`_folder_ids queries by `folder_id.`

**What should be** : CREATE INDEX `idx_images_folder_id` ON images(`folder_id`);

**Why?**: Without an index, viewing a specific folder requires scanning the entire images table.

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