feat(tests): add unit and integration test suite (node:test)

Adds 215 tests across three tiers using node:test + node:assert/strict
(no new test framework dependencies):

- test/tools/ — middleware and utility tests (token, hash, authenticate,
  jsonBody, convertError, validators)
- test/shared/ — DbInterface/BackendDbInterface contract suites reusable
  across backends (users, abodes, residents, apikeys, sessions, auth)
- test/backends/sqlite/ — SQLite-private tests (sql builder, WrappedDb,
  migrator) + shared suites via SqliteInterface
- test/backends/api/ — ApiInterface unit tests + shared suites via a
  live Koa server backed by SQLite

Also fixes four bugs uncovered by the tests:
- ApiInterface: path params leaked into query string (slice(1) fix)
- ApiInterface: calling res.json() on 204 No Content responses
- SqliteInterface.updateResident: missing comma in SET clause
- WrappedBetterSqlite3Db: readonly:undefined rejected by better-sqlite3

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/helpers/sqlite.ts

@@ -0,0 +1,18 @@
+import { getWrappedDb } from "../../src/db/sqlite/impl/index.js";
+import { SqliteMigrator } from "../../src/db/sqlite/SqliteMigrator.js";
+import { SqliteInterface } from "../../src/db/sqlite/SqliteInterface.js";
+import type { WrappedDb } from "../../src/db/sqlite/impl/types.js";
+
+export interface TestDb {
+  db: SqliteInterface;
+  wrapped: WrappedDb;
+  close(): void;
+}
+
+export async function createTestDb(): Promise<TestDb> {
+  const wrapped = getWrappedDb("node", ":memory:", {});
+  const migrator = new SqliteMigrator(wrapped);
+  await migrator.migrateTo(3);
+  const db = new SqliteInterface(wrapped);
+  return { db, wrapped, close: () => wrapped.destroy() };
+}