(feat) add entity and trigger views to signal instance page

[aliciamatsumoto]

Adds views to the signal instance table to provide basic summary statistics per entity and per snooze filter.

Backend changes

• updates the SignalFilterRead Pydantic model to include the backreference to the associated signal. Uses SignalBase to avoid recursive dependency.

Frontend changes

• Refactors TableInstance.vue into four files: a parent file of the same name that sets the active table view, and three dedicated table files for each respective grouping.
• Moves the signal instance table into a dedicated instance/ directory alongside the now three signal views.
• Creates a MultiSignalPopover component to display the (potentially multiple) signals that may be associated with a given signal filter.
• Creates a dedicated store.js in the instance/ directory to handle API calls dedicated to the different views.