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/models/tenant.py

@@ -0,0 +1,47 @@
+"""
+Modello Tenant – ogni organizzazione cliente del SaaS.
+"""
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import Boolean, DateTime, Index, Integer, String, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.database import Base
+
+
+class Tenant(Base):
+    __tablename__ = "tenants"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    slug: Mapped[str] = mapped_column(String(63), nullable=False, unique=True)
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    plan: Mapped[str] = mapped_column(String(50), nullable=False, default="starter")
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    max_mailboxes: Mapped[int] = mapped_column(Integer, nullable=False, default=5)
+    max_users: Mapped[int] = mapped_column(Integer, nullable=False, default=10)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
+    )
+
+    # Relazioni
+    users: Mapped[list["User"]] = relationship(  # noqa: F821
+        "User", back_populates="tenant", cascade="all, delete-orphan"
+    )
+    mailboxes: Mapped[list["Mailbox"]] = relationship(  # noqa: F821
+        "Mailbox", back_populates="tenant", cascade="all, delete-orphan"
+    )
+
+    __table_args__ = (
+        Index("idx_tenants_slug", "slug"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<Tenant {self.slug!r} ({self.plan})>"