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/label.py

@@ -0,0 +1,46 @@
+"""
+Modelli Label e MessageLabel – tagging messaggi.
+"""
+
+import uuid
+
+from sqlalchemy import CHAR, ForeignKey, Index, String, UniqueConstraint
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class Label(Base):
+    __tablename__ = "labels"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    tenant_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("tenants.id", ondelete="CASCADE"), nullable=False
+    )
+    name: Mapped[str] = mapped_column(String(100), nullable=False)
+    color: Mapped[str | None] = mapped_column(CHAR(7), nullable=True)  # hex #RRGGBB
+
+    __table_args__ = (
+        UniqueConstraint("tenant_id", "name", name="uq_label_name_tenant"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<Label {self.name!r}>"
+
+
+class MessageLabel(Base):
+    __tablename__ = "message_labels"
+
+    message_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("messages.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    label_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("labels.id", ondelete="CASCADE"),
+        primary_key=True,
+    )