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/agent/route.ts

@@ -0,0 +1,85 @@
+// @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 Agent Chat API (Agentic Mode)
+ * POST /api/ai/agent
+ * Body: { message, context, language, conversationHistory }
+ * 
+ * Only teachers and admins can use the agentic AI.
+ */
+
+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 use AI agent
+    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 use the AI agent' },
+        { status: 403 }
+      );
+    }
+
+    const { schoolId } = session.user;
+    if (!schoolId) {
+      return NextResponse.json({ error: 'No school assigned' }, { status: 403 });
+    }
+
+    const body = await request.json();
+    const { message, context = 'teacher-assistant', language = 'de', conversationHistory = [] } = body;
+
+    if (!message || typeof message !== 'string') {
+      return NextResponse.json({ error: 'Message is required' }, { status: 400 });
+    }
+
+    // Build system prompt based on context
+    const systemPrompts: Record<string, string> = {
+      'teacher-assistant': `Du bist ein KI-Assistent für Lehrkräfte an einer deutschen Schule.
+Du hilfst bei Unterrichtsvorbereitung, Bewertungsvorschlägen, Kompetenzanalyse und Differenzierung.
+Du kennst das deutsche Schulsystem (Grundschule, Gymnasium) und die kompetenzorientierte Beurteilung.
+Antworte auf Deutsch, es sei denn es wird explizit anders verlangt.
+Sei konkret, praktisch und praxisnah.`,
+      'lesson-planner': `Du bist ein Experte für Unterrichtsplanung nach dem kompetenzorientierten Ansatz.
+Du erstellst strukturierte Unterrichtsentwürfe mit Phasen, Lernzielen und Differenzierung.`,
+      'assessment-helper': `Du bist ein Experte für Leistungsbewertung.
+Du hilfst bei der Erstellung von Bewertungsrastern, Kompetenzchecklisten und Notenbegründungen.`,
+    };
+
+    const systemPrompt = systemPrompts[context] || systemPrompts['teacher-assistant'];
+
+    // Build messages array
+    const messages = [
+      { role: 'system', content: systemPrompt },
+      ...conversationHistory.map((msg: any) => ({
+        role: msg.role,
+        content: msg.content,
+      })),
+      { role: 'user', content: message },
+    ];
+
+    // Generate using z-ai-web-dev-sdk
+    const zai = await ZAI.create();
+    const response = await zai.chat.completions.create({
+      messages,
+      thinking: { type: 'disabled' },
+    });
+
+    const responseContent = response.choices?.[0]?.message?.content || 'Ich konnte keine Antwort generieren.';
+
+    return NextResponse.json({ response: responseContent });
+  } catch (error) {
+    console.error('AI Agent error:', error);
+    return NextResponse.json(
+      { error: 'Internal server error' },
+      { status: 500 }
+    );
+  }
+}