ci: capture backend and frontend logs for E2E failures (#481)

When an E2E test flakes in CI, the backend and frontend stdout is gone
the moment the step exits. That makes a failed run extremely hard to
triage: you get the Playwright report but nothing from the services
underneath it. Redirect both dev servers to files under the workspace's
ci-logs/ directory and include that directory in the failure-case
artifact upload, so the report and the service logs travel together.

Log files live under $GITHUB_WORKSPACE rather than /tmp because
actions/upload-artifact@v4+ computes a common parent directory across
all path entries. Mixing repo-relative paths with /tmp paths makes '/'
the common root, which v4+ then strips from the artifact, silently
dropping the absolute entries. Keeping everything under the workspace
avoids that footgun.

`stdbuf -oL -eL` forces line-buffered output on both services. Node
stdout is block-buffered when redirected to a regular file, so a crash
or abort can lose the final few hundred lines - exactly the output you
need when debugging a CI failure.

Also remove the duplicated `E2E_USERNAME: admin` / `E2E_PASSWORD:
password123` env blocks from the e2e and update-screenshots jobs.
Those values were already the fallback defaults in e2e/helpers.ts
(`process.env.E2E_USERNAME ?? 'admin'`), so the env blocks were doing
nothing on the default path and created two sources of truth for the
same literals. e2e/helpers.ts remains the canonical source; callers
that want to override the creds still set E2E_USERNAME / E2E_PASSWORD
as job-level env.

.github/workflows/ci.yml

@@ -153,11 +153,8 @@ jobs:
 
       - name: Run E2E tests
         run: npx playwright test
-        env:
-          E2E_USERNAME: admin
-          E2E_PASSWORD: password123
 
-      - name: Upload E2E report
+      - name: Upload E2E report and service logs
         if: failure()
         uses: actions/upload-artifact@v7
         with:
@@ -165,6 +162,7 @@ jobs:
           path: |
             e2e/report/
             test-results/
+            ci-logs/
           retention-days: 7
 
   # ---------------------------------------------------------------------------
@@ -194,9 +192,6 @@ jobs:
 
       - name: Capture screenshots
         run: npx playwright test e2e/screenshots.spec.ts --project=chromium
-        env:
-          E2E_USERNAME: admin
-          E2E_PASSWORD: password123
 
       - name: Open / update screenshots PR
         id: create-pr