Add milk carton missing member feature (!haveyouseenthem, !missing)

Generates milk carton style "missing person" PNG images using gg library
with embedded Go fonts. Tracks inactive members via daily_activity, with
configurable thresholds (MISSING_THRESHOLD_DAYS, MISSING_MAX_DAYS,
MISSING_MIN_MESSAGES, MISSING_EXCLUDE_USERS). Rate limited to 1 carton
per room per day. Derives flavor text characteristics from user stats
and sentiment data. Avatar fetching with 3-tier fallback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.go

@@ -102,6 +102,9 @@ func main() {
 	moviesPlugin := plugin.NewMoviesPlugin(client)
 	registry.Register(moviesPlugin)
 
+	// Community
+	registry.Register(plugin.NewMilkCartonPlugin(client, ratePlugin))
+
 	// Tracking (passive)
 	registry.Register(plugin.NewAchievementsPlugin(client, registry))
 	registry.Register(plugin.NewReactionsPlugin(client))