Migrate the API to native ESM and vitest; pin Node 26

Prep PR for the admin auth module (docs/plan-admin-auth.md step 1).
better-auth 1.7 ships ESM only, so the API moves off CommonJS:

- "type": "module", module nodenext, target ES2024, .js suffixes on all
  relative imports, require('mariadb'|'cors') replaced by imports, and
  export= packages (winston, app-root-path, bcrypt) consumed via default
  imports. The logger now uses appRoot.path explicitly.
- TypeScript 5.9, @types/node 26, tslint removed. Node 26 pinned via
  engines and .nvmrc (Plesk runs 26).
- Jest 28 + ts-jest replaced by vitest 5. Eight test files depend on
  hoisted module mocks with static imports and resetModules + require,
  which Jest's ESM mode does not support; vitest keeps them nearly
  verbatim. Coverage via @vitest/coverage-v8 (lcov), Sonar generic report
  via vitest-sonar-reporter, so sonar-project.properties is unchanged.
  vitest.config.ts sets FEEDBACK_IP_SALT so the suite passes without a
  local .env.
- dotenv 8 -> 16 and axios 0.24 -> 1.x: their old typings are not
  resolvable under nodenext.
- autoCommit: false dropped from the pool configs; it is not a mariadb
  connector option and was silently ignored.

tsc clean, 96/96 tests green, compiled app boots and serves /, /docs and
CORS under Node ESM.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/feedback/submissions.service.test.ts

@@ -1,4 +1,5 @@
-import {MAX_ANSWER_ROWS, validateAnswers, validateGuestBook, validateNewsletter} from '../../src/models/feedback/public/submissions.service';
+import {describe, it, expect} from 'vitest';
+import {MAX_ANSWER_ROWS, validateAnswers, validateGuestBook, validateNewsletter} from '../../src/models/feedback/public/submissions.service.js';
 
 type QuestionLookup = Map<number, {eventQuestionId: number; questionId: number; type: 'SONG_PICK' | 'SONG_RATING' | 'FREE_TEXT'; label: string; position: number}>;