FFC Broker Limits to avoid storms and outages

[Steve-Mcl]

Glad to see this. It partly covers my concerns raised on slack: https://flowforgeworkspace.slack.com/archives/C032Q63FGG1/p1741721144396989

In my opinion, there is more we can/should do.

NOTES:

1. This does not stop someone poking the value into the API (this can be a follow up)
2. This does not handle any existing entries set for mosquitto (this might be a follow up)
3. This does not stop users connecting to other popular public brokers (this can be a follow up)
4. We might wish to consider a global topic level limit (discussed in the linked slack thread) (this can be a follow up)

Originally posted by @Steve-Mcl in https://github.com/FlowFuse/flowfuse/issues/5389#issuecomment-2775944227