feat(file): conceal file resource existence from other users

- feat: cross-user file access returns not-found instead of
  permission-denied
- fix: repository delete now only affects active rows and returns
  ErrNotFound when no row changes; repeated deletes yield not-found
- feat: parent directory operations (list, upload, create-dir, move)
  conceal ownership of other user's directories
- test: update handler, service, repository, and integration tests to
  assert not-found behavior for cross-user and missing file operations

internal/handler/file_test.go

@@ -588,6 +588,16 @@ func TestFileHandler_Delete(t *testing.T) {
 		t.Errorf("status = %d, want %d; body = %s", rec.Code, http.StatusNoContent, rec.Body.String())
 	}
 
+	// Repeated deletes return not found.
+	req = httptest.NewRequest(http.MethodDelete, "/api/v1/files/"+uploaded.ID, nil)
+	req.Header.Set(testUserIDHeader, "user1")
+	rec = httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("repeated delete status = %d, want %d; body = %s", rec.Code, http.StatusNotFound, rec.Body.String())
+	}
+
 	// Verify gone.
 	req = httptest.NewRequest(http.MethodGet, "/api/v1/files/"+uploaded.ID, nil)
 	req.Header.Set(testUserIDHeader, "user1")
@@ -599,7 +609,20 @@ func TestFileHandler_Delete(t *testing.T) {
 	}
 }
 
-func TestFileHandler_ForbiddenAccess(t *testing.T) {
+func TestFileHandler_DeleteMissingReturnsNotFound(t *testing.T) {
+	r := setupFileRouter(t)
+
+	req := httptest.NewRequest(http.MethodDelete, "/api/v1/files/missing-file", nil)
+	req.Header.Set(testUserIDHeader, "user1")
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want %d; body = %s", rec.Code, http.StatusNotFound, rec.Body.String())
+	}
+}
+
+func TestFileHandler_CrossUserAccessReturnsNotFound(t *testing.T) {
 	r := setupFileRouter(t)
 
 	// Upload as user1.
@@ -624,7 +647,13 @@ func TestFileHandler_ForbiddenAccess(t *testing.T) {
 	rec = httptest.NewRecorder()
 	r.ServeHTTP(rec, req)
 
-	if rec.Code != http.StatusForbidden {
-		t.Errorf("status = %d, want %d", rec.Code, http.StatusForbidden)
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusNotFound)
+	}
+	if !strings.Contains(rec.Body.String(), "file not found") {
+		t.Errorf("body = %s, want file not found message", rec.Body.String())
+	}
+	if strings.Contains(rec.Body.String(), "access denied") {
+		t.Errorf("body leaks authorization result: %s", rec.Body.String())
 	}
 }