refactor(handlers): standardize logging using request attributes helper

- Replace manual IP logging with the `withRequestLogAttrs` helper in authentication handlers.
- Add user activity logging for API documentation and login page views.
- Clean up log calls to use variadic expansion of request attributes.

backend/libs/handlers/api_docs.go

@@ -21,6 +21,18 @@ type apiDocsData struct {
 }
 
 func (a *App) APIDocs(w http.ResponseWriter, r *http.Request) {
+	user, loggedIn := a.currentUser(r)
+	actor := "anonymous"
+	if loggedIn {
+		actor = "user"
+	}
+	a.logger.Info("api docs viewed", withRequestLogAttrs(r,
+		"source", "page",
+		"severity", "user_activity",
+		"code", 2501,
+		"actor", actor,
+		"user_id", user.ID,
+	)...)
 	a.renderPage(w, r, http.StatusOK, "api.html", web.PageData{
 		Title:       "API documentation",
 		Description: "Curl and ShareX upload examples for Warpbox.",