Add dynamic OG image generation for rich iMessage/social previews

Generates a branded 1200x630 PNG per event with title, date/time/location
using Go's image package and a custom bitmap font. Supports BBQ_BASE_URL
env var for absolute URLs required by link preview crawlers.

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

handlers.go

@@ -124,6 +124,7 @@ type EventPageData struct {
 	Rsvps      []db.Rsvp
 	TotalGoing int64
 	IsAdmin    bool
+	BaseURL    string
 }
 
 func (s *Server) loadEventPage(r *http.Request, slug string, isAdmin bool) (*EventPageData, error) {
@@ -181,6 +182,7 @@ func (s *Server) loadEventPage(r *http.Request, slug string, isAdmin bool) (*Eve
 		Rsvps:      rsvps,
 		TotalGoing: totalGoing,
 		IsAdmin:    isAdmin,
+		BaseURL:    s.baseURL,
 	}, nil
 }