Phase 7: pinning, plan versions, solver budget + UX polish

Backend (school-service):
  - tt_solution gains parent_solution_id (self-FK, ON DELETE SET NULL)
    and seconds_limit columns via ALTER TABLE IF NOT EXISTS.
  - CreateTimetableSolutionRequest accepts optional parent_solution_id
    and seconds_limit (5-600s) with binding validation.
  - CreateSolution checks parent ownership before INSERT so users can't
    fork another tenant's plan.
  - New PUT /timetable/lessons/:id/pin endpoint; ownership enforced via
    the lesson's solution.created_by_user_id JOIN.

Solver:
  - Lesson.pinned now carries @PlanningPin so Timefold leaves locked
    cells untouched during the search.
  - build_problem() takes optional parent_solution_id; if set, copies
    pinned (class_id, subject_id, day, period, room) tuples onto fresh
    Lesson objects via greedy first-fit matching. Surplus pinned rows
    from curriculum changes are silently dropped.
  - _build_factory(seconds) replaces the module-level factory so each
    job honours its tt_solution.seconds_limit override.
  - persist_solution writes lesson.pinned back so subsequent re-solves
    inherit it.

Frontend (studio-v2):
  - SolutionList grows three knobs in the create-form: Basieren auf
    (parent dropdown, only completed solutions, disabled when none),
    Sekunden-Limit (5-600), and the existing Name.
  - PlanView cells get a pin/unpin button with optimistic update and
    rollback on error. Pinned cells gain an amber ring.
  - types.ts + api.ts mirror the new fields; lessonsApi.pin(id, bool).
  - HelpPanel: collapsible 6-step Bedienungsanleitung explaining the
    setup-to-plan workflow. Anchored at the top of /stundenplan above
    the dev token banner.
  - page.tsx switches to the same gradient + animated-blob background
    used on /korrektur so /stundenplan stops looking like a slate-900
    test page.
  - JWT dev banner gets a step-by-step explanation of how to grab the
    token from DevTools and a non-blocking success indicator (no more
    alert()).

Tests:
  - school-service: 6 new validator cases for parent_solution_id +
    seconds_limit boundaries. 73 subtests total, all green.
  - studio-v2: mockSchoolApi adds PUT /lessons/:id/pin route. 5 new
    Playwright tests across two suites (parent-selector visibility +
    options, seconds-limit input, pin button render, pin-icon flip).
    Existing tests adjusted to the new help panel + JWT banner wording.

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

studio-v2/app/stundenplan/_components/plan/PlanView.tsx

@@ -2,7 +2,7 @@
 
 import { useState, useEffect, useCallback, useMemo } from 'react'
 import { useTheme } from '@/lib/ThemeContext'
-import { solutionsApi, subjectsApi } from '@/lib/stundenplan/api'
+import { solutionsApi, subjectsApi, lessonsApi } from '@/lib/stundenplan/api'
 import type { TimetableLesson, TimetableSubject } from '@/app/stundenplan/types'
 
 interface PlanViewProps {
@@ -109,6 +109,19 @@ export function PlanView({ solutionId }: PlanViewProps) {
   const cellLesson = (day: number, periodIdx: number): TimetableLesson | undefined =>
     visibleLessons.find(l => l.day_of_week === day && l.period_index === periodIdx)
 
+  const togglePin = useCallback(async (lesson: TimetableLesson) => {
+    // Optimistic update so the lock icon flips immediately even if the
+    // server is slow.
+    setLessons(prev => prev.map(l => l.id === lesson.id ? { ...l, pinned: !l.pinned } : l))
+    try {
+      await lessonsApi.pin(lesson.id, !lesson.pinned)
+    } catch (e) {
+      // Revert on failure and surface the error.
+      setLessons(prev => prev.map(l => l.id === lesson.id ? { ...l, pinned: lesson.pinned } : l))
+      setError(e instanceof Error ? e.message : 'Pin fehlgeschlagen')
+    }
+  }, [])
+
   const cardClass = isDark ? 'bg-white/10 border-white/20 text-white' : 'bg-white/80 border-black/10 text-slate-900'
   const selectClass = isDark ? 'bg-white/10 border-white/20 text-white' : 'bg-white border-slate-300 text-slate-900'
 
@@ -177,11 +190,23 @@ export function PlanView({ solutionId }: PlanViewProps) {
                     return (
                       <td key={d.v} className="px-2 py-1">
                         <div
-                          className="rounded-md p-2 text-xs space-y-0.5"
+                          className={`rounded-md p-2 text-xs space-y-0.5 relative ${lesson.pinned ? 'ring-2 ring-amber-400/70' : ''}`}
                           style={{ backgroundColor: color + (isDark ? '40' : '30'), borderLeft: `3px solid ${color}` }}
                           data-testid={`cell-${d.v}-${idx}`}
                         >
-                          <div className="font-semibold">{lesson.subject_name || '?'}</div>
+                          <button
+                            onClick={() => togglePin(lesson)}
+                            data-testid={`pin-${lesson.id}`}
+                            title={lesson.pinned ? 'Lesson loesen' : 'Lesson anpinnen'}
+                            className={`absolute top-1 right-1 text-xs leading-none px-1 py-0.5 rounded ${
+                              lesson.pinned
+                                ? 'text-amber-300 hover:text-amber-200'
+                                : 'opacity-30 hover:opacity-100'
+                            }`}
+                          >
+                            {lesson.pinned ? '🔒' : '📌'}
+                          </button>
+                          <div className="font-semibold pr-5">{lesson.subject_name || '?'}</div>
                           {perspective !== 'class' && lesson.class_name && (
                             <div className="opacity-80">{lesson.class_name}</div>
                           )}