fix(upload): tipo da magic bytes, non da Content-Type dichiarato (#36 punto 2)

Il Content-Type del client non era verificato: un file "x.html" dichiarato
image/png veniva salvato e poi servito per estensione come text/html,
eseguito dal browser. Nuova detect_file_type() (PNG/JPEG/GIF/WebP/PDF) in
files.py, usata anche dal logo in settings.py; l'estensione salvata segue
sempre il tipo rilevato, mai il nome file dichiarato dal client. Il logo
rifiuta SVG a prescindere (puo' portare script). Upload letti a blocchi da
1MB, interrotti appena superano il limite invece di bufferizzare tutto
prima di controllare la dimensione.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/api/routers/settings.py

@@ -1,6 +1,4 @@
 """System settings router - read/update settings, upload company logo."""
-from pathlib import Path
-
 from fastapi import APIRouter, Depends, File, HTTPException, UploadFile, status
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -8,6 +6,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from src.backend.config import settings
 from src.backend.database import get_db
 from src.backend.api.middleware.api_key import get_current_user, require_admin_user
+from src.backend.api.routers.files import ALLOWED_IMAGE_TYPES, detect_file_type
 from src.backend.models.orm.setting import SystemSetting
 from src.backend.models.orm.user import User
 
@@ -126,35 +125,45 @@ async def upload_company_logo(
     """Upload company logo.
 
     Saves logo to uploads/logos/ and updates company_logo_path setting.
-    Only admins can upload the logo.
+    Only admins can upload the logo. The type is determined from the file's
+    magic bytes, not the client-declared Content-Type (#36 point 2). SVG is
+    refused outright: it can itself carry script content, and there is no
+    safe way to serve it inline for a logo shown on every page.
     """
-    # Validate file type (must be image)
-    allowed_types = {"image/jpeg", "image/png", "image/gif", "image/webp", "image/svg+xml"}
-    if file.content_type not in allowed_types:
-        raise HTTPException(
-            status_code=status.HTTP_400_BAD_REQUEST,
-            detail=f"File type {file.content_type} not allowed. Must be an image.",
-        )
-
-    # Read file content
-    content = await file.read()
+    # Read in chunks, stopping as soon as the size limit is crossed instead
+    # of buffering an oversized body into memory first.
+    max_bytes = settings.max_upload_size_mb * 1024 * 1024
+    buffer = bytearray()
+    chunk_size = 1024 * 1024
+    while True:
+        chunk = await file.read(chunk_size)
+        if not chunk:
+            break
+        buffer.extend(chunk)
+        if len(buffer) > max_bytes:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail=f"File size exceeds maximum {settings.max_upload_size_mb}MB",
+            )
+    content = bytes(buffer)
     file_size = len(content)
 
-    # Validate file size
-    max_bytes = settings.max_upload_size_mb * 1024 * 1024
-    if file_size > max_bytes:
+    # Validate file type from the actual bytes (SVG never matches: refused)
+    detected = detect_file_type(content)
+    if detected is None or detected[0] not in ALLOWED_IMAGE_TYPES:
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
-            detail=f"File size {file_size} bytes exceeds maximum {settings.max_upload_size_mb}MB",
+            detail="File type not allowed. Must be a recognised image (SVG is not accepted).",
         )
+    _detected_mime, detected_ext = detected
 
     # Create logos directory
     logos_dir = settings.upload_path / "logos"
     logos_dir.mkdir(parents=True, exist_ok=True)
 
-    # Use a fixed filename for the company logo
-    file_extension = Path(file.filename).suffix
-    logo_filename = f"company_logo{file_extension}"
+    # Fixed filename for the company logo, extension forced to what was
+    # actually detected (not what the client claimed or the filename said).
+    logo_filename = f"company_logo{detected_ext}"
     logo_path = logos_dir / logo_filename
 
     # Write file