chore(e2e): add ESLint + Prettier; fix real findings; dedupe BASE

The Playwright suite had no linter and no formatter — only tsc. Add flat-config ESLint
(typescript-eslint, type-aware) and Prettier (2-space, matching the suite's style).

Rules keep the ones that catch real TEST bugs and drop the noise:
  - no-floating-promises KEPT — an un-awaited request/assertion can let a test end before it runs,
    passing vacuously. It caught one: the SSE reader loop in sse-listener is now explicitly `void`.
  - no-unused-vars KEPT — caught three dead bindings (an unused adminToken fixture arg, an unused
    `api` arg, an unused JPEG_MAGIC import), all removed.
  - no-explicit-any OFF — all test code; `any` is the honest type for an untyped res.json() body or
    a page.evaluate() return.
  - no-empty-pattern OFF — Playwright's dependency-free fixtures are `async ({}, use) => {}`.

Refactor: `const BASE = process.env.E2E_FRONTEND_URL ?? '...'` was redeclared verbatim in 23
files — extracted to helpers/env.ts and imported, so a port/scheme change is one edit not a sweep.

Then `prettier --write`. Verified: eslint clean, tsc clean, prettier clean, desktop suite 210
passed / 1 skipped. (One mobile spec flaked once under retries:0 — a pre-existing cross-test
reflow-timing vector from the flakiness audit, not this change: the each-key edit is stable across
16 isolated runs and a clean full mobile re-run.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/fixtures/db.ts

@@ -39,11 +39,16 @@ export const db = {
 
   async expireSession(userId: string) {
     await withClient((c) =>
-      c.query(`UPDATE session SET expires_at = NOW() - interval '1 hour' WHERE user_id = $1`, [userId])
+      c.query(`UPDATE session SET expires_at = NOW() - interval '1 hour' WHERE user_id = $1`, [
+        userId,
+      ])
     );
   },
 
-  async setUploadCompressionStatus(uploadId: string, status: 'pending' | 'processing' | 'done' | 'failed') {
+  async setUploadCompressionStatus(
+    uploadId: string,
+    status: 'pending' | 'processing' | 'done' | 'failed'
+  ) {
     await withClient((c) =>
       c.query(`UPDATE upload SET compression_status = $2 WHERE id = $1`, [uploadId, status])
     );
@@ -113,7 +118,11 @@ export const db = {
    * Insert a pre-baked export job row to skip the (slow) real compression path. Stamped with the
    * event's CURRENT epoch so it counts as the live generation.
    */
-  async fakeExportJob(eventSlug: string, type: 'zip' | 'html', status: 'pending' | 'running' | 'done') {
+  async fakeExportJob(
+    eventSlug: string,
+    type: 'zip' | 'html',
+    status: 'pending' | 'running' | 'done'
+  ) {
     await withClient(async (c) => {
       const ev = await c.query<{ id: string; export_epoch: string }>(
         `SELECT id, export_epoch FROM event WHERE slug = $1`,