FR: Get notification on tag changes

### What are you trying to do?

I would like to be able to be **notified** on any - or specific machine - **tag changes**.

I had it now multiple times that machines "lost" their tags somehow. Due to upgrade, due to misconfiguration, due to re-authentication, due to different admins doing different things.

Since tags are such an integral part of ACLs, I would love to be notified when a tag is changed for any or a specific machine.

### How should we solve this?

This notification could be via e-mail such as "machine-x has been added the tags [a,b,c], tags [d,e,f] were removed" or something along those lines.

Of course different notification providers (e.g. Pushover, Discord etc.) would also be awesome, e-mail would just be the easy common denominator.

### What is the impact of not solving this?

I'm currently being annoyed by this lack of notification, because sometimes debugging a problem is really hard. I check the firewall (nope, all green), I check the router (that's not it), I check `tailscale ping` (works) and it is really hard to figure out that tailscale is actively blocking stuff I'm trying to do. Blocking it because of missing tags.

I could write more extensive tests in the ACL editor, but I can't test each machine / each use-case, right now I just need to think about ACLs that **could** be an issue due to misconfigured tags, unfortunately this is late in my problem-solving-process.

### Anything else?

_No response_