fix: A16 — ACL sur l'export et les pièces jointes (v7.3.4)

- `export.py` : `_load_page_or_404(request, page_id)` — 401 sans session,
  404 si `PermissionManager.can_view_page` refuse (les 4 formats d'export)
- `dashboard.py` : helper `_require_page_view` sur `GET /api/pages/{id}/download`
  et `GET /api/pages/{id}/file-content`
- +1 test de non-régression → suite **1026/1026**, `ruff check app tests` OK

app/routers/export.py

@@ -23,7 +23,15 @@ logger = logging.getLogger(__name__)
 router = APIRouter(tags=["export"], prefix="/api/export")
 
 
-def _load_page_or_404(page_id: int) -> dict:
+def _load_page_or_404(request: Request, page_id: int) -> dict:
+    """A16 : session obligatoire + `PermissionManager.can_view_page` — l'export ne
+    doit pas délivrer le contenu d'une page énumérable par id."""
+    from app.auth.session import SessionManager
+    from app.services.permission_manager import PermissionManager
+
+    user = SessionManager.decode_session(request.cookies.get("flowdeck_session", ""))
+    if not user or not user.get("id"):
+        raise HTTPException(status_code=401, detail="Authentication required")
     with get_conn() as conn:
         row = conn.execute(
             "SELECT * FROM pages WHERE id=? AND deleted_at IS NULL",
@@ -31,6 +39,8 @@ def _load_page_or_404(page_id: int) -> dict:
         ).fetchone()
     if not row:
         raise HTTPException(status_code=404, detail="Page not found")
+    if not PermissionManager(user["id"], bool(user.get("is_admin"))).can_view_page(page_id):
+        raise HTTPException(status_code=404, detail="Page not found")
     return dict(row)
 
 
@@ -52,7 +62,7 @@ def _safe_filename(page: dict, ext: str) -> str:
 
 @router.get("/markdown/{page_id}")
 async def export_markdown(page_id: int, request: Request):
-    page = _load_page_or_404(page_id)
+    page = _load_page_or_404(request, page_id)
     md = page_to_markdown(page)
     filename = _safe_filename(page, "md")
     headers = _download_header(filename, "text/markdown")
@@ -61,7 +71,7 @@ async def export_markdown(page_id: int, request: Request):
 
 @router.get("/html/{page_id}")
 async def export_html(page_id: int, request: Request):
-    page = _load_page_or_404(page_id)
+    page = _load_page_or_404(request, page_id)
     html = page_to_standalone_html(page)
     filename = _safe_filename(page, "html")
     headers = _download_header(filename, "text/html")
@@ -70,7 +80,7 @@ async def export_html(page_id: int, request: Request):
 
 @router.get("/pdf/{page_id}")
 async def export_pdf(page_id: int, request: Request):
-    page = _load_page_or_404(page_id)
+    page = _load_page_or_404(request, page_id)
     try:
         pdf_bytes = page_to_pdf_bytes(page)
     except ImportError:
@@ -85,7 +95,7 @@ async def export_pdf(page_id: int, request: Request):
 
 @router.get("/site/{page_id}")
 async def export_site(page_id: int, request: Request):
-    page = _load_page_or_404(page_id)
+    page = _load_page_or_404(request, page_id)
     site_bytes = build_static_site_bytes(page)
     title = _safe_filename(page, "site").replace(".site", "") or "flowdeck-site"
     filename = f"{title}_site.zip"