A background page never gets any content scripts injected. It's not an http://
page to begin with, but even then it's not possible to specify a match pattern for it.
If you need jQuery there (along with plugins), you need to list it as a background script:
"background": {
"scripts": ["jquery.min.js", "jAlert-v2.js", "background.js"]
},
Do note: the background page is invisible. Showing a custom alert on it won't help you anyhow. Looks like you're confised about how extensions work; start with the Overview page.
Also, you should learn to debug the background page.