Add tenant merge dry-run planner

backend/tests/tenantMergePlan.test.ts

@@ -0,0 +1,56 @@
+import assert from "node:assert/strict"
+import test from "node:test"
+
+import { buildTenantMergePlan } from "../src/utils/tenantMergePlan"
+
+test("classifies imports, existing rows, conflicts and global id collisions", () => {
+    const plan = buildTenantMergePlan({
+        units: [
+            { id: 1, name: "Stück", short: "Stk." },
+            { id: 2, name: "Monat", short: "Mon." },
+            { id: 3, name: "Stunde", short: "Std." },
+        ],
+        customers: [
+            { id: 10, tenant: 42, name: "Neu" },
+            { id: 11, tenant: 42, name: "Geändert" },
+        ],
+    }, {
+        units: [
+            { id: 1, name: "Stück", short: "Stk.", updated_at: "later" },
+            { id: 2, name: "Kilometer", short: "km" },
+            { id: 9, name: "Stunde", short: "h" },
+        ],
+        customers: [
+            { id: 11, tenant: 42, name: "Zieländerung" },
+        ],
+    }, {
+        units: { primaryKey: ["id"] },
+        customers: { primaryKey: ["id"] },
+    })
+
+    assert.deepEqual(plan.summary, {
+        import: 1,
+        existing: 1,
+        conflict: 2,
+        id_collision: 1,
+    })
+
+    assert.equal(plan.items.find((item) => item.label === "Monat")?.kind, "id_collision")
+    assert.equal(plan.items.find((item) => item.label === "Stunde")?.kind, "conflict")
+    assert.equal(plan.items.find((item) => item.label === "Geändert")?.defaultDecision, "target")
+})
+
+test("uses target as the safe default for two-way conflicts", () => {
+    const plan = buildTenantMergePlan({
+        accounts: [{ id: 5, accountChart: "skr03", number: "8400", label: "Quelle" }],
+    }, {
+        accounts: [{ id: 99, accountChart: "skr03", number: "8400", label: "Ziel" }],
+    }, {
+        accounts: { primaryKey: ["id"] },
+    })
+
+    assert.equal(plan.items[0].kind, "conflict")
+    assert.equal(plan.items[0].targetKey, "id=99")
+    assert.equal(plan.items[0].defaultDecision, "target")
+    assert.deepEqual(plan.items[0].differences.sort(), ["id", "label"])
+})