借款出借人增加员工与公司组合数据库唯一约束（09-17-company-loan-principal-repayment）

- 新迁移 20261011010000_loan_lender_employee_unique：(legal_entity_id, employee_id) 唯一，归档计入、无员工出借人不冲突；生产只读排查重复 0 组
- 触到约束时返回原有 loan.employee_lender_conflict，不报 500 或误报请求重复；员工行锁与代码检查保留
- 测试改为断言数据库拒绝模糊旧数据，补空员工共存与归档占用；变异 12 项含去索引与数据库兜底两种运行
- PRD 补本迁移回退说明；独立只读复核无阻断，发布窗口需重跑重复计数（AC59）

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/qa/company-loan-employee-mutations.cjs

@@ -16,7 +16,10 @@ module.exports = async function employeeMutations(h, runFixture) {
     // Restores the pre-fix fingerprint (updated_at selected and hashed).
     ['source_version_updated_at', [['status: true, archived_at: true,\n', 'status: true, archived_at: true, updated_at: true,\n'],
       ['"loan-employee-source-v2", legalEntityId, employee.id,', '"loan-employee-source-v2", legalEntityId, employee.id, employee.updated_at,']]],
-    ['legacy_duplicate', 'if (links.some((lender) => lender.id !== lenderId))', 'if (false)'],
+    // The code check is proven with the database constraint removed; with it in
+    // place the same mutant must be fully absorbed, reporting the same conflict.
+    ['legacy_duplicate', 'if (links.some((lender) => lender.id !== lenderId))', 'if (false)', { withoutConstraint: true }],
+    ['legacy_duplicate_db_backstop', 'if (links.some((lender) => lender.id !== lenderId))', 'if (false)', { expectAbsorbed: true }],
     ['employee_lock', 'if (unique.length) await tx.$queryRaw', 'if (false) await tx.$queryRaw'],
     ['employee_lock_fk_compatibility', 'ORDER BY id FOR NO KEY UPDATE', 'ORDER BY id FOR UPDATE'],
     ['in_flight', 'await assertLenderNotInFlight(tx, id);', ''],
@@ -25,7 +28,8 @@ module.exports = async function employeeMutations(h, runFixture) {
     ['required_audit', 'await auditLoan(tx, request, "LOAN_LENDER_EMPLOYEE_CREATED", created.id, {\n          ...requestBinding, employeeSourceRequestKey: input.requestKey,\n        });', ''],
   ];
   const results = [];
-  for (const [name, from, to] of mutations) {
+  const INDEX = 'loan_lenders_legal_entity_id_employee_id_key';
+  for (const [name, from, to, options = {}] of mutations) {
     const edits = Array.isArray(from) ? from : [[from, to]];
     let mutated = source;
     for (const [target, replacement] of edits) {
@@ -37,8 +41,24 @@ module.exports = async function employeeMutations(h, runFixture) {
     mutant._compile(mutated, filename);
     let failure;
     Object.assign(live, mutant.exports);
+    if (options.withoutConstraint) await h.prisma.$executeRawUnsafe(`DROP INDEX "${INDEX}"`);
     try { await runFixture(h); } catch (error) { failure = error; }
-    finally { Object.assign(live, original); }
+    finally {
+      Object.assign(live, original);
+      if (options.withoutConstraint) {
+        // Detach the duplicates this mutant let through, then restore the index.
+        await h.prisma.$executeRawUnsafe(`UPDATE loan_lenders SET employee_id = NULL WHERE id IN (SELECT id FROM (
+          SELECT id, row_number() OVER (PARTITION BY legal_entity_id, employee_id ORDER BY created_at, id) AS n
+          FROM loan_lenders WHERE employee_id IS NOT NULL) ranked WHERE n > 1)`);
+        await h.prisma.$executeRawUnsafe(`CREATE UNIQUE INDEX "${INDEX}" ON loan_lenders (legal_entity_id, employee_id)`);
+      }
+    }
+    if (options.expectAbsorbed) {
+      assert.ok(!failure, `mutation ${name} was not absorbed by the database constraint: ${failure?.message?.split('\n')[0]}`);
+      results.push({ name, absorbedByDatabase: true, witness: 'database_constraint_backstop' });
+      console.log(JSON.stringify({ stage: 'loan_employee_mutation', ...results.at(-1) }));
+      continue;
+    }
     assert.ok(failure, `mutation ${name} survived`);
     // A generic environment, SQL or parsing failure is not a killed guard.
     const wrongAcceptance = failure.code === 'ERR_ASSERTION' && failure.actual === 200 && [403, 409, 500].includes(failure.expected);