test: enforce per-directory coverage thresholds

entities + use-cases + controllers must hit 100% (95% branches).
Project-wide baseline remains 80/75/80/80. Tightening these directories
reflects the architectural intent: these are the pure-logic layers and
should be exhaustively tested.

Added @vitest/coverage-v8@^3 to core-typescript, auth, blog,
marketing-pages, and navigation devDependencies.

Excluded from coverage (legitimately untestable):
- src/di/bind-production.ts — InversifyJS startup bootstrap
- src/application/repositories/** — pure TypeScript interfaces
- src/application/services/** — pure TypeScript interfaces (auth)
- src/integrations/cms/** — declarative Payload CMS config
- src/entities/cookie.ts — pure type aliases (auth)
- src/ui/** — React Query helpers, integration-tested in apps

Tests added (6 new files / 1 extended):
- auth/src/entities/errors.test.ts — UnauthenticatedError, UnauthorizedError,
  AuthenticationError, InputParseError constructors
- blog/src/entities/errors.test.ts — ArticleNotFoundError (default + custom
  message), InputParseError
- marketing-pages/src/entities/errors.test.ts — PageNotFoundError, InputParseError
- marketing-pages/src/entities/site-settings.test.ts — siteSettingsSchema
  (valid, no description, empty name rejection)
- navigation/src/entities/header.test.ts — headerItemSchema and headerSchema
  validation paths
- navigation/src/.../payload-header.repository.test.ts — logo relation-object,
  scalar-id, null, and null-item-field branches for full branch coverage

Spec: §6.9

packages/auth/package.json

@@ -28,6 +28,7 @@
     "@repo/core-testing": "workspace:*",
     "@repo/core-typescript": "workspace:*",
     "@types/node": "^22.0.0",
+    "@vitest/coverage-v8": "^3.2.4",
     "vitest": "^3.1.0"
   }
 }