Admin-only CRUD over the named credential sets from Phase 0:
- GET /api/credentials list all sets grouped by service (NO secrets)
- POST /api/credentials create {service,label,payload}, validated
- PUT /api/credentials/<id> update label and/or payload (partial)
- DELETE /api/credentials/<id> delete (clears any profile selections → fallback)
All four are @admin_only (non-admin → 403), payloads validated via
core.credentials.store, secrets never returned to the browser. Additive — no
existing endpoint or behaviour changes.
Tests: real web_server app + Flask test client (8) — create/list/update/delete
roundtrip, payload never leaks in list, missing-field/unsupported-service/blank-
label/duplicate(409)/404 validation, and the non-admin 403 gate on every write.
Verified the web_server import coexists with the rest of the suite (175 mixed
tests pass).
