Phase 9c: Parent accounts, magic-link login + parent timetable view

Backend (school-service):
  - parent_account, parent_child, parent_magic_link, parent_session
    tables. Tokens are sha256-hashed in DB; raw goes back exactly
    once to the inviting teacher.
  - InviteParent upserts the parent account, links a child to a tt_
    class, mints a 7-day magic link. Returns the link path so the
    teacher can paste it into Matrix/Email.
  - RedeemMagicLink validates + marks used + mints a 30-day session,
    sets HttpOnly bp_parent_session cookie.
  - ParentSessionMiddleware reads the cookie and resolves the parent.
    Lives in its own router group /api/v1/parent — totally separate
    from the teacher JWT path.
  - ParentMe returns the account + list of children (with class name).
  - ParentTimetable returns the latest completed tt_solution's lessons
    for the requested child's class, with full authorization check
    (parent must own a child in that class).

Frontend (studio-v2):
  - lib/calendar/subject-i18n.ts maps 22 German subject names to 8
    parent locales (de/en/tr/ar/uk/ru/pl/fr). Falls back to German
    for custom subjects.
  - ParentManager component on the Schulkalender page lets the teacher
    invite parents via email + child name + class + language. Newly
    minted magic-link is shown with a copy-to-clipboard button.
  - app/api/parent/[...path]/route.ts proxies parent-side endpoints
    via the cookie so HttpOnly survives the Next.js round-trip.
  - /eltern/login?token=… redeems and redirects to /eltern.
  - /eltern shows a Wochengrid with German days + translated subject
    names in the parent's preferred language. Headings and weekday
    labels also localised (de/en/tr/ar/uk/ru/pl/fr).

Tests:
  - 3 new Go unit tests (random token, hash stability, invite-request
    validator). 83 subtests gesamt.
  - studio-v2: e2e/eltern.spec.ts mit 7 tests across ParentManager,
    /eltern/login, /eltern overview, subject-i18n end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

studio-v2/app/eltern/login/page.tsx

@@ -0,0 +1,44 @@
+'use client'
+
+import { Suspense, useEffect, useState } from 'react'
+import { useRouter, useSearchParams } from 'next/navigation'
+import { elternApi } from '@/lib/eltern/api'
+
+function LoginInner() {
+  const router = useRouter()
+  const search = useSearchParams()
+  const [error, setError] = useState<string | null>(null)
+  const [done, setDone] = useState(false)
+
+  useEffect(() => {
+    const token = search.get('token')
+    if (!token) {
+      setError('Kein Token in der URL. Bitte den Link aus der Einladung verwenden.')
+      return
+    }
+    elternApi.redeem(token)
+      .then(() => { setDone(true); setTimeout(() => router.replace('/eltern'), 800) })
+      .catch(e => setError(e instanceof Error ? e.message : 'Login fehlgeschlagen'))
+  }, [router, search])
+
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-gradient-to-br from-indigo-900 via-purple-900 to-pink-800 text-white">
+      <div className="max-w-md w-full mx-4 p-6 rounded-2xl bg-white/10 border border-white/20 backdrop-blur-xl" data-testid="eltern-login">
+        <h1 className="text-2xl font-semibold mb-3">Eltern-Login</h1>
+        {!error && !done && <p className="opacity-80">Pruefe Token …</p>}
+        {done && <p className="text-emerald-200">Erfolgreich angemeldet. Weiterleitung …</p>}
+        {error && (
+          <div className="p-3 rounded-lg bg-red-500/20 border border-red-500/40 text-red-200 text-sm">{error}</div>
+        )}
+      </div>
+    </div>
+  )
+}
+
+export default function ElternLoginPage() {
+  return (
+    <Suspense fallback={null}>
+      <LoginInner />
+    </Suspense>
+  )
+}