feat(04-01): install new deps + scaffold API test harness with Wave-0 RED stubs

- Add react-router@7, @dnd-kit/core, @dnd-kit/sortable, fractional-indexing to PWA
- Add fractional-indexing to API (rank generation server-side)
- ioredis NOT added (in-memory EventEmitter per RESEARCH Plan 02 justification)
- Create apps/api/test/setup.ts with afterEach DB cleanup for list tables
- Wire test.setupFiles in apps/api/vitest.config.ts
- Add 4 Wave-0 RED stub test files (LIST-01/02/03/04, D-04, D-11, D-07)
- All stubs run as todo, not import-error

apps/api/test/setup.ts

@@ -0,0 +1,37 @@
+/**
+ * Vitest global test setup for apps/api.
+ *
+ * Establishes shared test infrastructure for API tests:
+ *   - DB pool access via the existing client.ts (DB_HOST/DB_NAME from env)
+ *   - Per-test cleanup for list tables (truncate between tests so state is isolated)
+ *
+ * Usage:
+ *   This file is referenced in vitest.config.ts via test.setupFiles.
+ *   Pure-logic tests (listEmitter, fractional rank) do NOT require DB — the
+ *   cleanup function is a no-op when the tables are empty.
+ *
+ * Environment:
+ *   Set DB_HOST, DB_USER, DB_PASSWORD, DB_NAME in the test environment.
+ *   Tests run against a local MariaDB dev database; production data is never touched.
+ */
+
+import { afterEach } from 'vitest'
+import { db } from '../src/db/client.js'
+import { lists, listItems, listShares } from '../src/db/schema.js'
+
+/**
+ * Truncate list tables in FK-safe order after each test.
+ * list_items and list_shares have FK to lists; delete children first.
+ * Called automatically via afterEach — no per-test setup needed.
+ */
+afterEach(async () => {
+  try {
+    // Delete child rows first to avoid FK constraint violations
+    await db.delete(listItems)
+    await db.delete(listShares)
+    await db.delete(lists)
+  } catch {
+    // DB may not be available in pure-unit test runs (no DB_HOST configured).
+    // Swallow the error — pure-logic tests do not need cleanup.
+  }
+})