fix: Restore all files lost during destructive rebase

A previous `git pull --rebase origin main` dropped 177 local commits,
losing 3400+ files across admin-v2, backend, studio-v2, website,
klausur-service, and many other services. The partial restore attempt
(660295e2) only recovered some files.

This commit restores all missing files from pre-rebase ref 98933f5e
while preserving post-rebase additions (night-scheduler, night-mode UI,
NightModeWidget dashboard integration).

Restored features include:
- AI Module Sidebar (FAB), OCR Labeling, OCR Compare
- GPU Dashboard, RAG Pipeline, Magic Help
- Klausur-Korrektur (8 files), Abitur-Archiv (5+ files)
- Companion, Zeugnisse-Crawler, Screen Flow
- Full backend, studio-v2, website, klausur-service
- All compliance SDKs, agent-core, voice-service
- CI/CD configs, documentation, scripts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/classroom_engine/database.py

@@ -0,0 +1,49 @@
+"""
+Database Configuration fuer Classroom Engine.
+
+Stellt PostgreSQL-Anbindung fuer Session-Persistenz bereit.
+Sessions ueberleben Server-Neustarts (Feature f22).
+"""
+import os
+from sqlalchemy import create_engine
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import sessionmaker
+
+# Database URL from environment (nutzt gleiche DB wie Backend)
+# Korrigiert postgres:// zu postgresql:// fuer SQLAlchemy 2.0 Kompatibilitaet
+_raw_url = os.getenv(
+    "DATABASE_URL",
+    "postgresql://breakpilot:breakpilot123@localhost:5432/breakpilot"
+)
+# SQLAlchemy 2.0 erfordert "postgresql://" statt "postgres://"
+DATABASE_URL = _raw_url.replace("postgres://", "postgresql://", 1) if _raw_url.startswith("postgres://") else _raw_url
+
+# Engine configuration
+engine = create_engine(
+    DATABASE_URL,
+    pool_pre_ping=True,
+    pool_size=5,
+    max_overflow=10,
+    echo=os.getenv("SQL_ECHO", "false").lower() == "true"
+)
+
+# Declarative Base
+Base = declarative_base()
+
+# Session factory
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+
+def get_db():
+    """Database dependency for FastAPI endpoints."""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+
+
+def init_db():
+    """Erstellt alle Tabellen (fuer Entwicklung)."""
+    from . import db_models  # Import models to register them
+    Base.metadata.create_all(bind=engine)