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/lib/schulkalender/api.ts

@@ -7,6 +7,7 @@ import { getStundenplanToken } from '@/lib/stundenplan/api'
 import type {
   PublicEvent, SchoolCalendarConfig, UpsertSchoolCalendarConfig,
   SchoolEvent, CreateSchoolEvent, SchoolYearRolloverResult,
+  ParentInviteListItem, InviteParentRequest, InviteParentResponse,
 } from '@/app/schulkalender/types'
 
 async function apiFetch<T>(endpoint: string, options: RequestInit = {}): Promise<T> {
@@ -49,4 +50,11 @@ export const calendarApi = {
         new_year_end: newYearEnd,
       }),
     }),
+
+  // Phase 9c: parent invitations
+  listParents: () => apiFetch<ParentInviteListItem[]>('/calendar/parents'),
+  inviteParent: (data: InviteParentRequest) =>
+    apiFetch<InviteParentResponse>('/calendar/parents/invite', { method: 'POST', body: JSON.stringify(data) }),
+  deleteParentChild: (childId: string) =>
+    apiFetch<void>(`/calendar/parents/children/${childId}`, { method: 'DELETE' }),
 }