feat: background photo system — random Lanzarote landscapes behind CV grid

Dev-only toggle button enables/disables photo backgrounds. Photos are
auto-discovered from static/images/backgrounds/ and randomly selected
on each page load. Production is unaffected — no button, no photo.

internal/handlers/cv_helpers.go

@@ -352,6 +352,19 @@ func (h *CVHandler) prepareTemplateData(lang string) (map[string]interface{}, er
 		}
 	}
 
+	// Scan background photos (dev only)
+	var bgPhotos []string
+	if !isProduction {
+		bgDir := filepath.Join(c.DirStatic, "images", "backgrounds")
+		entries, _ := os.ReadDir(bgDir)
+		for _, e := range entries {
+			name := e.Name()
+			if !e.IsDir() && (strings.HasSuffix(name, ".jpg") || strings.HasSuffix(name, ".png") || strings.HasSuffix(name, ".webp")) {
+				bgPhotos = append(bgPhotos, "/static/images/backgrounds/"+name)
+			}
+		}
+	}
+
 	// Prepare template data
 	data := map[string]interface{}{
 		"CV":                &cv,
@@ -366,6 +379,7 @@ func (h *CVHandler) prepareTemplateData(lang string) (map[string]interface{}, er
 		"AlternateEN":       "https://juan.andres.morenorub.io/?lang=en",
 		"AlternateES":       "https://juan.andres.morenorub.io/?lang=es",
 		"ChatEnabled":       h.chatEnabled,
+		"BgPhotos":          bgPhotos,
 	}
 
 	return data, nil