Add internal grant endpoint + temp-password / force-change flow

- /api/internal/grant: secret-auth endpoint for payment-router to
  provision access on a paid order — find-or-create user (emailVerified,
  temp password, mustChangePassword), enroll by course slug list,
  AccessLog, delivery email. Idempotent by order_id.
- User.mustChangePassword flag (+ migration); (student) layout redirects
  flagged users to /change-password (forced first-login change).
- email.ts: sendCourseGrantEmail (temp password for new buyers).
- middleware: /api/internal is public (own secret auth).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/middleware.ts

@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getSessionCookie } from "better-auth/cookies";
 
-const PUBLIC_ROUTES = ["/login", "/register", "/verify-email", "/forgot-password", "/reset-password", "/api/auth", "/api/register", "/maintenance"];
+const PUBLIC_ROUTES = ["/login", "/register", "/verify-email", "/forgot-password", "/reset-password", "/api/auth", "/api/register", "/api/internal", "/maintenance"];
 
 export function middleware(request: NextRequest) {
   const { pathname } = request.nextUrl;