refactor(queue): support multiple handlers per queue

[william-p]

This PR refactors the internal message queue to support registering multiple handlers per queue instead of a single one.

Previously, calling RegisterHandler would overwrite the existing handler. Now, each call appends to a list of handlers, and all registered handlers are invoked for every message.

[nb: I am not a Go developer but I have (tried) to intelligently use AI to help me]

[LinkinStars]

Using multiple handlers seems like a sound design approach. Perhaps you could elaborate on specific use cases, as currently the entire system typically only registers handlers once—scenarios requiring multiple registrations haven't yet arisen.

[william-p]

Today each queue has a single consumer (e.g. only the badge system listens to the event queue), but the event queue is fed by ~15 call sites across the codebase (question/answer/comment CRUD, votes, user updates…). This makes it a natural event bus that additional consumers — audit logging, search reindexing, cache invalidation, analytics — should be able to hook into without replacing the existing handler.

The goal of this PR is to add custom event handler.
Was useful for my Webhook feature (https://github.com/william-p/answer/commits/featWebhooks/ / #1509) for catching all events of question/answer (and others ...) life cycle.

The same applies to the notification and activity queues. Generalizing to multi-handler removes this limitation with minimal changes.