feature request: Replace SSE with React Query Polling

[panteLx]

Checklist

• I checked that this feature has not been requested before
• I checked that this feature is not in the "wontfix / not planned" list
• This feature will benefit the majority of users

Problem Description / Use Case

Unified real-time sync across entire app (Calendar Grid, Admin Panel, all Dialogs/Sheets)

Proposed Solution

No response