Disable CDN caching for HTML, add Quicklink prefetching

- Change Cache-Control from s-maxage=31536000 to max-age=0,must-revalidate
  for all HTML pages and sitemap (fixes stale content after settings change)
- Remove Cloudflare PurgeURLs call since we're not caching HTML at CDN
- Add Quicklink for client-side prefetching on public blog pages
- Make main branch deploy automatic (remove manual trigger)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/server/build.go

@@ -162,15 +162,5 @@ func (s *Server) rebuildSite(ctx context.Context, tenantID string, db *sql.DB, h
 		}
 	}
 
-	if s.cloudflare.IsConfigured() {
-		urls := make([]string, len(pages))
-		for i, p := range pages {
-			urls[i] = baseURL + p.path
-		}
-		if err := s.cloudflare.PurgeURLs(ctx, urls); err != nil {
-			slog.Error("rebuildSite: purge cache", "error", err)
-		}
-	}
-
 	slog.Info("rebuildSite: complete", "tenantID", tenantID, "pages", len(pages))
 }