fix(deps): bump next 15.0.3 → 16.2.6 to clear trivy CVEs

trivy fs scan failed the M0.2 CI gate on the skeleton commit because
next 15.0.3 has 9 known vulns (CRITICAL CVE-2025-29927 auth bypass in
middleware, plus 7 HIGH advisories). 16.2.6 is current latest and
covers every fixed-version range trivy listed.

Side effects of the major bump:
- next 16 dropped 'next lint' — switched the lint script to call eslint
  directly ('eslint . --max-warnings 0').
- eslint-config-next 16 ships flat-config exports natively, so
  eslint.config.mjs imports core-web-vitals + typescript directly
  (no FlatCompat shim, no @eslint/eslintrc dep).
- Typed vi.fn<typeof fetch>() in tenant-registry.test to satisfy
  stricter tuple inference under the new types.

All 4 gates green locally:
  pnpm lint / typecheck / test --coverage (100% on src/lib) / build

Refs: M5.1 (skeleton)

CHANGELOG.md

@@ -6,7 +6,8 @@ Generated section is appended on release tag via `git-cliff` (see `.gitea/workfl
 ## [Unreleased]
 
 ### Added
-- feat(app): Next.js 15 + Auth.js v5 skeleton with host→slug middleware, tenant context layout, OIDC sign-in flow
+- chore(deps): bump next + eslint-config-next to 16.2.6 to clear trivy CVEs (CVE-2025-29927 critical + 7 highs in next 15.0.3)
+- feat(app): Next.js 16 + Auth.js v5 skeleton with host→slug middleware, tenant context layout, OIDC sign-in flow
 -
 
 ### Changed