feat: Fase 1 – Fondamenta complete (backend FastAPI + auth + permessi)

- docker-compose.yml: PostgreSQL 16, Redis 7, MinIO, Nginx
- backend FastAPI: struttura monorepo, config pydantic-settings
- modelli SQLAlchemy: tutti i modelli (tenants, users, mailboxes, messages, archival, permissions, labels, audit_log)
- migrazione Alembic 0001: schema completo in pure SQL
- auth API: login JWT, refresh token rotation, logout, 2FA TOTP (setup/verify/disable)
- CRUD utenti: lista, crea, modifica, reset password, soft delete
- permessi granulari (Fase 1-A): mailbox_permissions, assegna/revoca/lista
- CRUD tenant: gestione super-admin
- sicurezza: AES-256-GCM cifratura credenziali IMAP/SMTP, bcrypt password
- RLS PostgreSQL: isolamento multi-tenant per request
- seed sviluppo: tenant demo + admin + operator
- test unit: security (bcrypt, JWT, AES), auth_service
- test integration: auth endpoints, users endpoints
- CI GitHub Actions: lint (ruff), test (pytest), build Docker, security scan
- infra: nginx.conf, redis.conf
- Makefile con comandi make dev/test/migrate/seed

Definition of Done:
✅ Login, refresh token e TOTP funzionanti
✅ make dev porta in piedi tutto lo stack locale
✅ CI configurata

backend/app/database.py

@@ -0,0 +1,52 @@
+"""
+Configurazione database – engine SQLAlchemy async e session factory.
+"""
+
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import get_settings
+
+settings = get_settings()
+
+# Engine async con pool connection
+engine = create_async_engine(
+    settings.database_url,
+    echo=settings.app_debug,          # log SQL in development
+    pool_size=10,
+    max_overflow=20,
+    pool_pre_ping=True,               # verifica connessione prima di usarla
+    pool_recycle=3600,                # ricicla connessioni ogni ora
+)
+
+# Session factory
+AsyncSessionLocal = async_sessionmaker(
+    bind=engine,
+    class_=AsyncSession,
+    expire_on_commit=False,
+    autocommit=False,
+    autoflush=False,
+)
+
+
+class Base(DeclarativeBase):
+    """Base class per tutti i modelli SQLAlchemy."""
+    pass
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    """
+    Dependency FastAPI: restituisce una sessione DB per ogni request.
+    La sessione viene chiusa automaticamente al termine del request.
+    """
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise
+        finally:
+            await session.close()