Restructure: Move final 16 root files into packages (backend-lehrer)

classroom/ (+2): state_engine_api, state_engine_models
vocabulary/ (2): api, db
worksheets/ (2): api, models
services/  (+6): audio, email, translation, claude_vision, ai_processor, story_generator
api/        (4): school, klausur_proxy, progress, user_language

Only main.py + config.py remain at root. 16 shims added.

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

backend-lehrer/worksheets/__init__.py

@@ -0,0 +1,37 @@
+# Worksheets Module
+# worksheets/api.py    — API router (generate MC, cloze, mindmap, quiz)
+# worksheets/models.py — Pydantic models and helpers
+
+from .api import router
+from .models import (
+    ContentType,
+    GenerateRequest,
+    MCGenerateRequest,
+    ClozeGenerateRequest,
+    MindmapGenerateRequest,
+    QuizGenerateRequest,
+    BatchGenerateRequest,
+    WorksheetContent,
+    GenerateResponse,
+    BatchGenerateResponse,
+    parse_difficulty,
+    parse_cloze_type,
+    parse_quiz_types,
+)
+
+__all__ = [
+    "router",
+    "ContentType",
+    "GenerateRequest",
+    "MCGenerateRequest",
+    "ClozeGenerateRequest",
+    "MindmapGenerateRequest",
+    "QuizGenerateRequest",
+    "BatchGenerateRequest",
+    "WorksheetContent",
+    "GenerateResponse",
+    "BatchGenerateResponse",
+    "parse_difficulty",
+    "parse_cloze_type",
+    "parse_quiz_types",
+]