Replace imghdr image validation

[Mirochill]

Fixes #67.

Summary:

• remove the imghdr import from main.py
• validate downloaded image files using local header checks for common image formats
• preserve the existing None result for unreadable files so the caller still removes invalid downloads

Validation:

• git diff --check HEAD~1..HEAD
• Static search confirmed imghdr is no longer imported or used.
• Not run locally.