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/core/pagination.py

@@ -0,0 +1,56 @@
+"""
+Utility per paginazione standardizzata nelle API.
+"""
+
+from typing import Generic, TypeVar
+
+from pydantic import BaseModel, Field
+
+T = TypeVar("T")
+
+DEFAULT_PAGE_SIZE = 25
+MAX_PAGE_SIZE = 100
+
+
+class PaginationParams(BaseModel):
+    page: int = Field(default=1, ge=1, description="Numero di pagina (1-based)")
+    page_size: int = Field(
+        default=DEFAULT_PAGE_SIZE,
+        ge=1,
+        le=MAX_PAGE_SIZE,
+        description=f"Elementi per pagina (max {MAX_PAGE_SIZE})",
+    )
+
+    @property
+    def offset(self) -> int:
+        return (self.page - 1) * self.page_size
+
+    @property
+    def limit(self) -> int:
+        return self.page_size
+
+
+class PaginatedResponse(BaseModel, Generic[T]):
+    """Risposta paginata generica."""
+    items: list[T]
+    total: int
+    page: int
+    page_size: int
+    pages: int
+
+    @classmethod
+    def create(
+        cls,
+        items: list[T],
+        total: int,
+        params: PaginationParams,
+    ) -> "PaginatedResponse[T]":
+        import math
+        pages = math.ceil(total / params.page_size) if params.page_size > 0 else 0
+        return cls(
+            items=items,
+            total=total,
+            page=params.page,
+            page_size=params.page_size,
+            pages=pages,
+        )