test: testing overhaul -- CI e2e gates, parallel suites, generated matrices, mutation testing (#215)

Closes the "e2e never runs in CI" hole. Adds per-PR e2e smoke gate,
nightly full-suite workflows, parallel vitest forks (per-fork DBs),
Playwright parallel/serial/visual projects against production builds,
metadata-generated test suites (drift guards, hostile inputs, format
matrix, pairwise settings, property-based fuzz), Stryker mutation
testing, Schemathesis API fuzz, coverage ratchet, and fixes for three
session-poisoning bugs that caused 200+ serial-bucket failures.

Bug fix included: favicon/split/bulk-rename could hang clients forever
when ZIP streaming failed after reply.hijack().

docker/docker-compose.test.yml

@@ -29,7 +29,14 @@ services:
       context: ..
       dockerfile: docker/Dockerfile.test
     container_name: SnapOtter-test-e2e
-    command: ["sh", "-c", "npx playwright install --with-deps chromium && pnpm test:e2e"]
+    # Visual project is excluded: linux baselines are managed by the
+    # update-visual-baselines workflow, not generated inside the container.
+    command:
+      [
+        "sh",
+        "-c",
+        "npx playwright install --with-deps chromium && pnpm playwright test --project=chromium && pnpm playwright test --project=chromium-serial --workers=1",
+      ]
     environment:
       - NODE_ENV=test
       - AUTH_ENABLED=true