feat(core-shared): grace-purge soft-deleted rows + boot registration

The retention purge job gated its whole body on activeRetention while
every collection declares only postDeletion, and no app ever called
registerRetentionPurgeJobs — retention was dead end to end (audit
findings A2 + A3). The DSR soft delete now stamps a deletedAt tombstone
on postDeletion collections (kept distinct from processingRestrictedAt
so an Art. 18 restriction never feeds the purge), the job grace-purges
tombstoned rows past postDeletion.duration with the declared action,
core-cms injects the tombstone field + Payload task definitions, and
bindAllProduction enqueues the first purge cycle at boot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/core-shared/src/payload/retention-purge/tombstone.test.ts

@@ -0,0 +1,83 @@
+import { describe, it, expect } from "vitest";
+import type { CollectionConfig } from "payload";
+import {
+  RETENTION_TOMBSTONE_FIELD,
+  hasPostDeletionPolicy,
+  withRetentionTombstone,
+} from "@/payload/retention-purge/tombstone";
+
+function makeCollection(custom?: Record<string, unknown>): CollectionConfig {
+  return {
+    slug: "things",
+    custom,
+    fields: [{ name: "title", type: "text" }],
+  } as CollectionConfig;
+}
+
+const postDeletionRetention = {
+  retention: {
+    purgeSchedule: "daily",
+    postDeletion: {
+      duration: "P30D",
+      trigger: "after-deletion",
+      action: "hard-delete",
+    },
+  },
+};
+
+describe("hasPostDeletionPolicy", () => {
+  it("is true when custom.retention.postDeletion is declared", () => {
+    expect(hasPostDeletionPolicy(makeCollection(postDeletionRetention))).toBe(
+      true,
+    );
+  });
+
+  it("is false without retention or postDeletion", () => {
+    expect(hasPostDeletionPolicy(makeCollection())).toBe(false);
+    expect(
+      hasPostDeletionPolicy(
+        makeCollection({ retention: { purgeSchedule: "daily" } }),
+      ),
+    ).toBe(false);
+  });
+});
+
+describe("withRetentionTombstone", () => {
+  it("appends the deletedAt field to postDeletion collections", () => {
+    const result = withRetentionTombstone(
+      makeCollection(postDeletionRetention),
+    );
+    const names = result.fields.map((f) => (f as { name?: string }).name);
+    expect(names).toContain(RETENTION_TOMBSTONE_FIELD);
+    const tombstone = result.fields.find(
+      (f) => (f as { name?: string }).name === RETENTION_TOMBSTONE_FIELD,
+    ) as { type?: string; index?: boolean };
+    expect(tombstone.type).toBe("date");
+    expect(tombstone.index).toBe(true);
+  });
+
+  it("returns collections without a postDeletion policy unchanged", () => {
+    const collection = makeCollection();
+    expect(withRetentionTombstone(collection)).toBe(collection);
+  });
+
+  it("does not duplicate an already-declared tombstone field", () => {
+    const collection = {
+      ...makeCollection(postDeletionRetention),
+      fields: [{ name: RETENTION_TOMBSTONE_FIELD, type: "date" }],
+    } as CollectionConfig;
+    const result = withRetentionTombstone(collection);
+    expect(
+      result.fields.filter(
+        (f) => (f as { name?: string }).name === RETENTION_TOMBSTONE_FIELD,
+      ),
+    ).toHaveLength(1);
+  });
+
+  it("does not mutate the input collection", () => {
+    const collection = makeCollection(postDeletionRetention);
+    const before = collection.fields.length;
+    withRetentionTombstone(collection);
+    expect(collection.fields.length).toBe(before);
+  });
+});