docs(prd): add 2026.5 platform upgrade umbrella PRD

Bundles six workstreams targeting the 2026.5.0 release: adopt dev-tooling
from JTL shipping-platform (ruff + pre-commit, .claude/ skills/rules),
consolidate modules/core + modules/graph + modules/admin into a single
auto-discovering modules/server/ package (scoped down from the full
issue #431 vision), finalize the Helm chart for EKS, add a Huey HTTP
producer layer so API and worker pods scale independently, add a
Playwright golden-path E2E gate in CI before release tags, and a recon
sub-PR that proposes any additional feature picks. Each workstream lands
as its own sub-PR into feat/2026.5; tag 2026.5.0 after all are green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: danh91