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 OSS modules/core + modules/graph + modules/admin into a
single auto-discovering modules/server/ package (scoped down from the
full issue #431 vision, NOT ee/insiders/modules/admin), 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 run the scripted subtree-sync workflow (per
SUBTREE_SYNC_WORKFLOW.md) to pull shipping-platform changes upstream.
Each workstream lands as its own sub-PR; tag 2026.5.0 after all merge.

Author: danh91