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>

src/db/sqlite/SqliteInterface.ts

@@ -319,7 +319,7 @@ export class SqliteInterface implements BackendDbInterface {
             UPDATE "residents"
             SET
               "updated_at" = datetime('now', 'localtime', 'subsec'),
-              "updated_by" = ${{ uuid: ctx.uid }}
+              "updated_by" = ${{ uuid: ctx.uid }},
               ${joinSql(updates, sql`, `)}
             WHERE
               "uid" = ${{ uuid: resident.uid }}