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/api/__init__.py

@@ -0,0 +1,6 @@
+# API Module — thin proxy routers and standalone API endpoints
+#
+# api/school.py        — Proxy to Go school-service
+# api/klausur_proxy.py — Proxy to klausur-service
+# api/progress.py      — Student learning progress tracking
+# api/user_language.py — User language preferences