test(imaging): add endpoint contract e2e and cross-browser render checks

The deploy gate only installs chromium, so imaging had no browser-level
coverage. Add two Playwright specs:

- e2e/imaging/avatar-endpoint.spec.ts: HTTP contract for the avatar
  (/api/imaging/avatar), default (/imaging) and badge endpoints — animated
  APNG for a figure with an effect, the trailing-`.0` regression figure,
  ETag/304 revalidation, and a 400 for a malformed figure. Runs in the
  deploy gate automatically via the default Playwright config.
- e2e/imaging/avatar-render.spec.ts: loads the avatar in a real browser and
  asserts it decodes at a non-zero size and animates. Navigates to the image
  URL directly so production CSP `upgrade-insecure-requests` cannot rewrite
  the request over a plain-HTTP host.

Add playwright.imaging.config.ts running both across chromium, firefox and
webkit, a `test:imaging` script, and a non-blocking CI job that installs all
three engines and checks the live site.

package.json

@@ -40,6 +40,7 @@
 		"test:news:real": "bash scripts/with-memory-cap.sh 8g node --import tsx e2e/news-real/run.ts",
 		"test:ui": "bash scripts/with-memory-cap.sh 8g playwright test --config playwright.ui.config.ts",
 		"test:ui:update": "bash scripts/with-memory-cap.sh 8g playwright test --config playwright.ui.config.ts --update-snapshots",
+		"test:imaging": "bash scripts/with-memory-cap.sh 8g playwright test --config playwright.imaging.config.ts",
 		"performance:report": "node scripts/performance-report.mjs",
 		"test:integration": "bash scripts/with-memory-cap.sh 8g vitest run --config vitest.integration.config.ts"
 	},