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/audit_log.py

@@ -0,0 +1,51 @@
+"""
+Modello AuditLog – immutabile per compliance e tracciabilità.
+"""
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import (
+    BigInteger,
+    DateTime,
+    ForeignKey,
+    Index,
+    String,
+    Text,
+    func,
+)
+from sqlalchemy.dialects.postgresql import INET, JSONB, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class AuditLog(Base):
+    __tablename__ = "audit_log"
+
+    id: Mapped[int] = mapped_column(BigInteger, primary_key=True, autoincrement=True)
+    tenant_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True
+    )
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id"), nullable=True
+    )
+    action: Mapped[str] = mapped_column(String(100), nullable=False)
+    resource_type: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    resource_id: Mapped[uuid.UUID | None] = mapped_column(UUID(as_uuid=True), nullable=True)
+    ip_address: Mapped[str | None] = mapped_column(INET, nullable=True)
+    user_agent: Mapped[str | None] = mapped_column(Text, nullable=True)
+    payload: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
+    outcome: Mapped[str] = mapped_column(String(20), nullable=False, default="success")
+    occurred_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+    __table_args__ = (
+        Index("idx_audit_tenant_date", "tenant_id", "occurred_at"),
+        Index("idx_audit_user", "user_id", "occurred_at"),
+        Index("idx_audit_action", "action"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<AuditLog {self.action!r} user={self.user_id} outcome={self.outcome!r}>"