feat(api,ui): PRP-40 — showcase planning + knowledge lifecycle

[w7-mgfcode]

Tracks PRP-40 implementation per PRPs/PRP-40-showcase-planning-knowledge-lifecycle.md.

Scope

Adds two new phases to the in-process /showcase demo pipeline (the showcase_rich scenario only):

• Planning (2 steps): scenario_simulate_and_save, multi_plan_compare
• Knowledge (3 steps): embedding_provider_probe, rag_index_subset, rag_retrieve_probe

Inserts both phases BEFORE the verify phase using relative anchors so PRP-39 (sibling, parallel) rebases cleanly.

Additive backend change: IndexProjectDocsRequest.path_prefix: str | None = None on app/features/rag/schemas.py so the showcase can index only the curated 5-file docs/user-guide corpus.

Acceptance (C1..C6)

• C1 — /visualize/planner shows showcase-price-cut-10pct + showcase-holiday-uplift after a green showcase_rich run; multi-plan compare row ranks by revenue_delta.
• C2 — /knowledge lists the 5 curated user-guide docs with non-zero chunk counts; UI semantic search returns hits.
• C3 — With every embedding provider unreachable, the 3 knowledge steps SKIP (not FAIL); pipeline still green.
• C4 — showcase_rich e2e wall-clock <= 240 s on dev host.
• C5 — Backend _phase_table() and frontend PHASE_DEFS.ts stay in lockstep.
• C6 — All five validation gates green.

Related

• Parent epic / sibling: PRP-39 (issue TBD, executing in parallel).
• Predecessor: feat(api,ui): showcase pipeline — richer data + V1/V2 modeling foundation (PRP-38) #309 (PRP-38).
• Docs PRP: docs(showcase): add PRP-39 and PRP-40 planning artifacts #313.

[w7-mgfcode]

Merged on dev via PR #317 — PRP-40 (showcase planning + knowledge lifecycle) is functionally complete. Closing as hygiene; epic PRP-38 → 41 finished in PR #323.