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/services/__init__.py

@@ -19,4 +19,12 @@ except (ImportError, OSError) as e:
     FileProcessor = None  # type: ignore
     _file_processor_available = False
 
+# Lazy-loaded service modules (imported on demand to avoid heavy deps at startup):
+# .audio          — TTS audio generation for vocabulary words
+# .email          — Email/SMTP service
+# .translation    — Batch vocabulary translation via Ollama
+# .claude_vision  — Claude Vision API for worksheet analysis
+# .ai_processor   — Legacy shim for ai_processor/ package
+# .story_generator — Story generation from vocabulary words
+
 __all__ = ["PDFService", "FileProcessor"]