feat(api,ui): showcase-completion E3 — advanced seed config mvp + store/product scope selection

[w7-mgfcode]

Sub-issue of #406 (umbrella: showcase workspace completion — the forecastlab control story). Parallel after Foundation (E1 #407).

Purpose

Advanced seed configuration MVP + scope selection: an additive nested-override schema on the seeder HTTP contract for a curated, allow-listed knob subset (store count, product count, date window/days, sparsity, promotion intensity, stockout intensity, noise/variance profile), an additive seed_overrides field on DemoRunRequest / the WS start frame, a store/product focus-pair selector with pre-run preview in the Showcase data phase, frontend + backend validation, and persistence of overrides + user-selected scope into the workspace row (E1 slots) so replay honors them verbatim.

Sub-tasks

To be decomposed via issue-to-subtasks when this epic is picked up.

[w7-mgfcode]

Delivered in #416 (merged to dev @ 195b384): allow-listed SeederOverrides on POST /seeder/generate, seed_overrides/user_scope on the demo start frame, advanced seed-config panel + focus-pair selector on /showcase, story-slot persistence + replay-verbatim (incl. e2e regression), docs updated (API_CONTRACTS / RUNBOOKS incident 29 / DOMAIN_MODEL slot schemas). Zero migrations — E1 #407 owns the schema.