feat(dashboard): add traffic flow sankey chart (#5465)

* feat(dashboard): add traffic flow sankey chart

Add dashboard flow APIs and a Sankey-based flow view with user, optional API key, model, and channel layers.\n\nReuse the dashboard VChart palette, add precise link/node tooltips and interactions, and cover filtering, layer ordering, color stability, and error states with tests.

* feat: build flow chart from quota data

---------

Co-authored-by: CaIon <i@caion.me>

model/task_cas_test.go

@@ -40,6 +40,7 @@ func TestMain(m *testing.M) {
 		&Token{},
 		&Log{},
 		&Channel{},
+		&QuotaData{},
 		&Ability{},
 		&TopUp{},
 		&SubscriptionPlan{},
@@ -62,6 +63,7 @@ func truncateTables(t *testing.T) {
 		DB.Exec("DELETE FROM tokens")
 		DB.Exec("DELETE FROM logs")
 		DB.Exec("DELETE FROM channels")
+		DB.Exec("DELETE FROM quota_data")
 		DB.Exec("DELETE FROM abilities")
 		DB.Exec("DELETE FROM top_ups")
 		DB.Exec("DELETE FROM subscription_orders")