feat(store): set trial_ends_at on tenant create

CreateTenant now defaults trial_ends_at to NOW() + 14 days when the
new tenant lands in status='trial'. Demo-kind tenants get
status='demo' (per PLATFORM_ARCHITECTURE.md §5d) and trial_ends_at
stays NULL — those flow through the M13.2 demo-provisioning path.

Both store implementations (Memory + Postgres) updated; tests assert
the 14-day window for customers and the absent end for demo kind.

Unblocks M12.1 (portal trial banner can render a real countdown).

Refs: M4.1 + M12.1

CHANGELOG.md

@@ -6,6 +6,7 @@ Generated section is appended on release tag via `git-cliff` (see `.gitea/workfl
 ## [Unreleased]
 
 ### Added
+- feat(store): CreateTenant defaults trial_ends_at to NOW()+14d for customer kind; demo kind gets status='demo' and no trial end
 - feat(keycloak): M4.3 — internal/keycloak adapter (Admin API: org create + IT_ADMIN invite + execute-actions-email + attribute sync). admin_email on POST /v1/tenants triggers KC provisioning; failures emit keycloak.provision_failed audit but don't roll back. POST /v1/internal/keycloak/claims resolves the current claim bundle for a tenant.
 - feat(api): M4.2 — full REST surface (tenants CRUD + lifecycle, catalog, entitlements, API keys w/ argon2 hashing, audit query). pgx-backed Postgres store; in-memory fallback when DATABASE_URL is empty. OpenAPI 3.1 spec at openapi.yaml with kin-openapi contract test.
 - feat(schema): M4.1 — golang-migrate migrations for tenants + tenant_projects + tenant_products + tenant_idp_config + api_keys + audit_log; cmd/migrate binary; testcontainers round-trip + seed + slug-constraint tests