ci: add @vitest/coverage-v8 to workspace packages; set REDIS_URL on build

CI unit-test runs vitest run --coverage in each workspace package, but only
apps/web declared the coverage-v8 dep. In pnpm workspaces deps aren't
hoisted across packages, so engine/staffing/api/application/shared need it
directly.

The build job also needs REDIS_URL because collecting page data for
/api/perf imports a module that throws if REDIS_URL is missing under
NODE_ENV=production. A placeholder value satisfies the check (no actual
Redis connection is made at build time).

packages/api/package.json

@@ -38,6 +38,7 @@
     "@capakraken/tsconfig": "workspace:*",
     "@types/node": "^22.10.2",
     "typescript": "^5.6.3",
-    "vitest": "^2.1.8"
+    "vitest": "^2.1.8",
+    "@vitest/coverage-v8": "^2.1.9"
   }
 }