fix: return 404 for missing organizations

[mikemikimike]

Summary

• map wrapped store.ErrNotFound errors from organization lookup to HTTP 404
• preserve HTTP 500 for genuine store failures
• add regression coverage for both status-code paths

Closes #2780

Why

lookupOrg preserves store.ErrNotFound with %w, but getOrganization previously mapped every lookup error to HTTP 500. Missing or deleted organizations are a normal not-found condition and should not be reported as server failures.

Tests

• go test ./api/pkg/server -run '^TestGetOrganization_ErrorStatus$' -count=1
• go test ./api/pkg/server -run '^Test(Get|Delete)Organization' -count=1
• git diff --check