Fix AI invalid JSON: add system prompt + robust JSON extraction

Opus 4.6 and other large models add preamble/thinking text before
JSON output. Fix: add system message enforcing JSON-only, strip
leading text before first {, trim trailing text after last }, and
log raw output on parse failure for debugging.

src/routes/learningAI.js

@@ -202,11 +202,19 @@ router.post('/ai-generate', upload.single('file'), async function(req, res) {
     var prompt = buildGeneratePrompt({ topic, docText, contentType, questionCount, refinement, wordCount, slideCount });
 
     var result = await callAI(
-      [{ role: 'user', content: prompt }],
+      [
+        { role: 'system', content: 'You are a medical education content generator. Return ONLY the requested JSON or Marp markdown — no preamble, no commentary, no code fences, no thinking. Start your response with { or --- as appropriate.' },
+        { role: 'user', content: prompt }
+      ],
       { model: model, temperature: 0.4, maxTokens: 6000 }
     );
 
     var raw = result.content.trim();
+    // Strip any leading text before the first { or --- (models sometimes add preamble)
+    if (contentType !== 'presentation' || parseInt(questionCount) > 0) {
+      var jsonStart = raw.indexOf('{');
+      if (jsonStart > 0) raw = raw.substring(jsonStart);
+    }
 
     // ── Presentation ──
     if (contentType === 'presentation') {
@@ -228,18 +236,27 @@ router.post('/ai-generate', upload.single('file'), async function(req, res) {
       return res.json({ success: true, contentType: 'presentation', marpMarkdown: marpMd, questions: [], model: result.model, docLength: docText.length });
     }
 
-    // Strip code fences if model added them anyway
+    // Strip code fences and any trailing text after JSON
     raw = raw.replace(/^```(?:json)?\s*/i, '').replace(/\s*```\s*$/, '');
+    // Trim trailing non-JSON text (e.g. "Here is the JSON..." after closing brace)
+    var lastBrace = raw.lastIndexOf('}');
+    if (lastBrace !== -1 && lastBrace < raw.length - 1) raw = raw.substring(0, lastBrace + 1);
 
     var parsed;
     try {
       parsed = JSON.parse(raw);
     } catch (e) {
-      var match = raw.match(/\{[\s\S]*\}/);
+      // Try extracting from first { to last }
-      if (match) {
+      var firstBrace = raw.indexOf('{');
-        try { parsed = JSON.parse(match[0]); }
+      var extractedJson = firstBrace >= 0 ? raw.substring(firstBrace, lastBrace + 1) : null;
-        catch (e2) { return res.status(500).json({ error: 'AI returned invalid JSON. Try again.' }); }
+      if (extractedJson) {
+        try { parsed = JSON.parse(extractedJson); }
+        catch (e2) {
+          console.error('[LearningAI] JSON parse failed. Raw (first 500):', raw.substring(0, 500));
+          return res.status(500).json({ error: 'AI returned invalid JSON. Try again.' });
+        }
       } else {
+        console.error('[LearningAI] No JSON found. Raw (first 500):', raw.substring(0, 500));
         return res.status(500).json({ error: 'AI returned invalid JSON. Try again.' });
       }
     }