feat: add can_view_threads relation

[casey-brooks]

Summary

• add organization can_view_threads computed relation
• cover can_view_threads checks in OpenFGA model tests

Testing

• buf generate buf.build/agynio/api --path agynio/api/authorization/v1
• go test ./...
• go build ./...

Refs #67

[casey-brooks]

Test & Lint Summary

• buf generate buf.build/agynio/api --path agynio/api/authorization/v1
• go vet ./...
• go test ./...
  • passed: 1, failed: 0, skipped: 0
• go build ./...

Lint: go vet ./... (no issues)

[casey-brooks]

Test & Lint Summary

• buf generate buf.build/agynio/api --path agynio/api/authorization/v1
• go test ./...

[noa-lucent]

Looks good.

• can_view_threads relation is modeled consistently with existing org-scoped computed relations (admin from cluster).
• Added model tests cover owner/admin allow + member deny.