Restrict AI Studio to teachers/admins only + Save to Lesson feature

Access Control:
- AI Studio view checks user role: only SCHOOL_ADMIN, SUPER_ADMIN, TEACHER, VICE_PRINCIPAL
- Students and parents see 'Kein Zugriff' (No Access) message
- AI Studio removed from student navigation
- API routes (/api/ai/snapgen, /api/ai/perchance, /api/ai/lesson-plan, /api/ai/agent) enforce role checks

New Features:
- 'Save to Lesson' dialog in Video and Image generation tabs
  - Pick Subject > Topic > Lesson from existing curriculum
  - Saves AI video/image URL to SubjectLesson record
  - Stores prompt, provider, timestamp, and teacher who generated
  - Students will see this content in homework and lesson materials
- AI Lesson Plan API route (/api/ai/lesson-plan)
- AI Agent Chat API route (/api/ai/agent) with context-aware system prompts
- SubjectLesson model: Added aiVideoUrl, aiImageUrl, aiThumbnailUrl, aiPrompt, aiProvider, aiGeneratedAt, aiGeneratedBy
- Homework model: Added aiLessonId, aiVideoUrl, aiImageUrl for linking AI content

Navigation:
- Removed duplicate AI Studio entry from teacher nav
- Removed AI Studio from student nav (students can't access)

src/app/api/ai/lesson-plan/route.ts

@@ -0,0 +1,96 @@
+// @ts-nocheck
+import { NextResponse } from 'next/server';
+import { db } from '@/lib/db';
+import { getSession } from '@/lib/auth';
+import ZAI from 'z-ai-web-dev-sdk';
+
+/**
+ * AI Lesson Plan Generation API
+ * POST /api/ai/lesson-plan
+ * Body: { subject, topic, classLevel, duration, language }
+ * 
+ * Only teachers and admins can generate lesson plans.
+ */
+
+export async function POST(request: Request) {
+  try {
+    const session = await getSession();
+    if (!session?.user) {
+      return NextResponse.json({ error: 'Not authenticated' }, { status: 401 });
+    }
+
+    // Only admin/teacher can generate lesson plans
+    const role = session.user.role;
+    if (role !== 'SCHOOL_ADMIN' && role !== 'SUPER_ADMIN' && role !== 'TEACHER' && role !== 'VICE_PRINCIPAL') {
+      return NextResponse.json(
+        { error: 'Only teachers and admins can generate lesson plans' },
+        { status: 403 }
+      );
+    }
+
+    const { schoolId } = session.user;
+    if (!schoolId) {
+      return NextResponse.json({ error: 'No school assigned' }, { status: 403 });
+    }
+
+    // Check AI settings
+    const aiSettings = await db.aISettings.findUnique({
+      where: { schoolId },
+    });
+
+    if (!aiSettings) {
+      return NextResponse.json(
+        { error: 'AI settings not configured' },
+        { status: 403 }
+      );
+    }
+
+    const body = await request.json();
+    const { subject, topic, classLevel, duration = 45, language = 'de' } = body;
+
+    if (!topic || typeof topic !== 'string') {
+      return NextResponse.json({ error: 'Topic is required' }, { status: 400 });
+    }
+
+    // Build the lesson plan prompt
+    const langText = language === 'de' ? 'Deutsch' : language === 'en' ? 'Englisch' : 'Türkçe';
+    const prompt = `Erstelle einen detaillierten Unterrichtsentwurf auf ${langText}.
+
+Fach: ${subject || 'Allgemein'}
+Thema: ${topic}
+Klassenstufe: ${classLevel || '3'}
+Dauer: ${duration} Minuten
+
+Struktur:
+1. Lernziele (3-5 Ziele)
+2. Einstieg/Motivation (5-10 Min.)
+3. Erarbeitungsphase (15-20 Min.)
+4. Übungsphase (10-15 Min.)
+5. Sicherung/Reflexion (5-10 Min.)
+6. Differenzierungsmöglichkeiten
+7. Materialien/Medien
+8. Hausaufgabe
+
+Bitte strukturiere die Antwort mit klaren Überschriften und Bulletpoints.`;
+
+    // Generate using z-ai-web-dev-sdk
+    const zai = await ZAI.create();
+    const response = await zai.chat.completions.create({
+      messages: [
+        { role: 'system', content: 'Du bist ein erfahrener Lehrer und Unterrichtsplaner. Du erstellst detaillierte, praxisnahe Unterrichtsentwürfe.' },
+        { role: 'user', content: prompt },
+      ],
+      thinking: { type: 'disabled' },
+    });
+
+    const content = response.choices?.[0]?.message?.content || 'Keine Antwort generiert.';
+
+    return NextResponse.json({ content });
+  } catch (error) {
+    console.error('AI Lesson Plan error:', error);
+    return NextResponse.json(
+      { error: 'Internal server error' },
+      { status: 500 }
+    );
+  }
+}