Adds max_posts INTEGER to campaign_subs (NULL = unlimited/evergreen).
Adds successful_post_count() query counting lifetime success records.
poster.py checks max_posts before the 7-day rolling dupe guard.
Root cause: campaign 2 fired 8 days after the last post (just outside the
7-day window), allowing a duplicate r/opensource pitch. Fix: set max_posts=1
on intro campaigns so the lifetime cap fires regardless of window.
