fix: handle age_limit=None in subscribe call (#140)

[hyldmh]

Description

Fix TypeError when age_limit is None in video metadata during subscribe.

Root Cause

ytcc subscribe crashes when YouTube/MakerTube returns age_limit: null in video JSON metadata. The line processed.get("age_limit", 0) returns None (not the default 0) when the key exists but is null, then None > config.ytcc.age_limit raises TypeError.

Fix

Changed processed.get("age_limit", 0) to (processed.get("age_limit") or 0) which converts None to 0 before comparison.

Fixes #140