Add interactive learning modules MVP (Phases 1-3.1)

New feature: After OCR vocabulary extraction, users can generate interactive
learning modules (flashcards, quiz, type trainer) with one click.

Frontend (studio-v2):
- Fortune Sheet spreadsheet editor tab in vocab-worksheet
- "Lernmodule generieren" button in ExportTab
- /learn page with unit overview and exercise type cards
- /learn/[unitId]/flashcards — Flip-card trainer with Leitner spaced repetition
- /learn/[unitId]/quiz — Multiple choice quiz with explanations
- /learn/[unitId]/type — Type-in trainer with Levenshtein distance feedback
- AudioButton component using Web Speech API for EN+DE TTS

Backend (klausur-service):
- vocab_learn_bridge.py: Converts VocabularyEntry[] to analysis_data format
- POST /sessions/{id}/generate-learning-unit endpoint

Backend (backend-lehrer):
- generate-qa, generate-mc, generate-cloze endpoints on learning units
- get-qa/mc/cloze data retrieval endpoints
- Leitner progress update + next review items endpoints

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/vocab_worksheet_api.py

@@ -2677,3 +2677,66 @@ async def load_ground_truth(session_id: str, page_number: int):
         gt_data = json.load(f)
 
     return {"success": True, "entries": gt_data.get("entries", []), "source": "disk"}
+
+
+# ─── Learning Module Generation ─────────────────────────────────────────────
+
+
+class GenerateLearningUnitRequest(BaseModel):
+    grade: Optional[str] = None
+    generate_modules: bool = True
+
+
+@router.post("/sessions/{session_id}/generate-learning-unit")
+async def generate_learning_unit_endpoint(session_id: str, request: GenerateLearningUnitRequest = None):
+    """
+    Create a Learning Unit from the vocabulary in this session.
+
+    1. Takes vocabulary from the session
+    2. Creates a Learning Unit in backend-lehrer
+    3. Optionally triggers MC/Cloze/QA generation
+
+    Returns the created unit info and generation status.
+    """
+    if request is None:
+        request = GenerateLearningUnitRequest()
+
+    if session_id not in _sessions:
+        raise HTTPException(status_code=404, detail="Session not found")
+
+    session = _sessions[session_id]
+    vocabulary = session.get("vocabulary", [])
+
+    if not vocabulary:
+        raise HTTPException(status_code=400, detail="No vocabulary in this session")
+
+    try:
+        from vocab_learn_bridge import create_learning_unit, generate_learning_modules
+
+        # Step 1: Create Learning Unit
+        result = await create_learning_unit(
+            session_name=session["name"],
+            vocabulary=vocabulary,
+            grade=request.grade,
+        )
+
+        # Step 2: Generate modules if requested
+        if request.generate_modules:
+            try:
+                gen_result = await generate_learning_modules(
+                    unit_id=result["unit_id"],
+                    analysis_path=result["analysis_path"],
+                )
+                result["generation"] = gen_result
+            except Exception as e:
+                logger.warning(f"Module generation failed (unit created): {e}")
+                result["generation"] = {"status": "error", "reason": str(e)}
+
+        return result
+
+    except ImportError:
+        raise HTTPException(status_code=501, detail="vocab_learn_bridge module not available")
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
+    except RuntimeError as e:
+        raise HTTPException(status_code=502, detail=str(e))