feat: wire admin time-blocker routes

Add GET/POST/DELETE /api/admin/time-blockers routes so admin
blockers (staff meetings, holidays, late starts) can be created
and managed via the API.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/main.go

@@ -218,6 +218,12 @@ func main() {
 				r.Get("/", notifications.GetNotifications)
 				r.Post("/{id}/acknowledge", notifications.AcknowledgeNotification)
 			})
+
+			r.Route("/admin/time-blockers", func(r chi.Router) {
+				r.Get("/", scheduling.ListTimeBlockers)
+				r.Post("/", scheduling.CreateTimeBlocker)
+				r.Delete("/{id}", scheduling.DeleteTimeBlocker)
+			})
 		})
 	})