feat(api,ui): PRP-39 — showcase decision + portfolio lifecycle

[w7-mgfcode]

Tracks PRP-39 implementation. Parent: #295.

Adds 3 new steps to the existing decision phase (champion_compat_compare, stale_alias_trigger, safer_promote_flow) and a new portfolio phase (batch_preset) to the showcase-rich demo pipeline. See PRPs/PRP-39-showcase-decision-portfolio-lifecycle.md for the full contract.

Slice B of the four-PRP /showcase upgrade epic (PRP-38..41). Depends on PRP-38 (#309) merged.

[w7-mgfcode]

Merged on dev via PR #318 — PRP-39 (showcase decision + portfolio lifecycle) is functionally complete. Closing as hygiene; epic PRP-38 → 41 finished in PR #323.