fix(seeder): make phase2 enrichment idempotent

[w7-mgfcode]

Problem

POST /seeder/phase2-enrichment (app/features/seeder/service.py phase2_enrichment(...); route at app/features/seeder/routes.py:313) inserts into replenishment_event, exogenous_signal, and sales_returns without conflict handling. A second call against an already-enriched database raises IntegrityError on uq_exogenous_signal_per_store (constraint name confirmed during the PRP-38 dogfood), and the unhandled exception surfaces as HTTP 500 instead of a clean RFC 7807 problem+json.

Repro

docker compose up -d
uv run alembic upgrade head
# Seed once, then enrich twice.
curl -s -X POST http://localhost:8123/seeder/generate -d '{"scenario":"showcase_rich","seed":42,"force":true}' -H 'Content-Type: application/json'
curl -s -X POST http://localhost:8123/seeder/phase2-enrichment -H 'Content-Type: application/json' -d '{}'   # → 200 (first call succeeds)
curl -s -X POST http://localhost:8123/seeder/phase2-enrichment -H 'Content-Type: application/json' -d '{}'   # → 500 (IntegrityError: uq_exogenous_signal_per_store)

Scope

The bug is reachable only on a manual second call. The showcase pipeline (POST /demo/run with scenario=showcase_rich) always seeds first inside the same run before calling phase2_enrichment, so the pipeline path never hits the duplicate state.

Fix options (~10-20 LOC)

1. Idempotent insert. Add .on_conflict_do_nothing() to the three Phase-2 inserts (replenishment_event, exogenous_signal, sales_returns) so a repeat call is a no-op. Cleanest, matches the app/features/ingest/service.py upsert idiom.
2. Clean 422. Catch IntegrityError in the service, raise UnprocessableEntityError("Phase 2 already enriched") → RFC 7807 422. Cheaper but explicit-failure UX (the operator must drop the existing enrichment to re-run).

Recommendation: option 1 plus a route test that calls the endpoint twice and asserts both responses are 200.

Notes

• Discovered during the PRP-38 browser dogfood (see PR feat(api,ui): showcase pipeline richer data and v2 foundation (#309) #310 body).
• Low priority; pipeline path unaffected.
• No prerequisites — independent of PRP-39 / PRP-40 / PRP-41.

[w7-mgfcode]

Fixed by PR #320 / merge b3ba1f4. Closing as hygiene — code is already on dev (commits 72a954e + 608d5a9).