feat: add the operations layer for declared plugin actions

src/backend/operations/constants.ts

@@ -0,0 +1,8 @@
+/** How many events or actions may lead to one another before the chain stops. */
+export const MAX_EVENT_DEPTH = 3;
+
+/** How long finished runs and events stay in history. */
+export const RETENTION_DAYS = 90;
+
+/** How long a cached table row or tile counts as fresh. */
+export const RESOURCE_FRESH_MS = 60_000;