fix(freedoc): widen freeform project_id to BIGINT for snowflake IDs

UI-11 POST /freeform-documents 500 when project_id exceeds int4.
Align Go types + goose 00063 with projects.id BIGINT.

internal/freedoc/create_document_test.go

@@ -59,6 +59,25 @@ func TestCreateDocument_ClientKeyIdempotentAndBootstrapsLedger(t *testing.T) {
 	assert.NotEqual(t, detail.Document.DocID, otherUser.Document.DocID)
 }
 
+func TestCreateDocument_AcceptsSnowflakeProjectID(t *testing.T) {
+	// projects.id is BIGINT; freeform must accept IDs above int4 max (regression: int4 encode 500).
+	const snowflakeProjectID int64 = 1789095399489
+	svc := freedoc.NewService(nil, nil)
+	detail, err := svc.CreateDocument(context.Background(), 42, freedoc.CreateFreeformDocRequest{
+		ProjectID:       snowflakeProjectID,
+		Title:           "snowflake.pdf",
+		ContentMarkdown: "# snowflake\n",
+		Space:           "my_private",
+	})
+	require.NoError(t, err)
+	assert.Equal(t, snowflakeProjectID, detail.Document.ProjectID)
+
+	list, err := svc.ListDocuments(context.Background(), 42, snowflakeProjectID, "my_private")
+	require.NoError(t, err)
+	require.Len(t, list, 1)
+	assert.Equal(t, snowflakeProjectID, list[0].ProjectID)
+}
+
 func TestCreateDocument_RejectsEmptyContent(t *testing.T) {
 	svc := freedoc.NewService(nil, nil)
 	_, err := svc.CreateDocument(context.Background(), 1, freedoc.CreateFreeformDocRequest{

internal/freedoc/service.go

@@ -525,7 +525,7 @@ func (s *Service) GetDocument(ctx context.Context, userID int, docID string) (*F
 }
 
 // ListDocuments 按空间与权限列出自由文档概要
-func (s *Service) ListDocuments(ctx context.Context, userID int, projectID int, space string) ([]FreeformDocSummary, error) {
+func (s *Service) ListDocuments(ctx context.Context, userID int, projectID int64, space string) ([]FreeformDocSummary, error) {
 	if space != "my_private" && space != "project_shared" && space != "trash" {
 		space = "my_private"
 	}

internal/freedoc/types.go

@@ -16,7 +16,7 @@ var (
 // FreeformDocument 自由私密/共享文档元数据
 type FreeformDocument struct {
 	DocID               string     `json:"doc_id"`
-	ProjectID           int        `json:"project_id"`
+	ProjectID           int64      `json:"project_id"`
 	OwnerUserID         int        `json:"owner_user_id"`
 	SourceMessageID     *int64     `json:"source_message_id,omitempty"`
 	SourceContentSHA256 string     `json:"source_content_sha256"`
@@ -53,7 +53,7 @@ type FreeformDocDetail struct {
 // FreeformDocSummary 列表项概要
 type FreeformDocSummary struct {
 	DocID            string    `json:"doc_id"`
-	ProjectID        int       `json:"project_id"`
+	ProjectID        int64     `json:"project_id"`
 	OwnerUserID      int       `json:"owner_user_id"`
 	Title            string    `json:"title"`
 	Space            string    `json:"space"`
@@ -66,14 +66,14 @@ type FreeformDocSummary struct {
 // CreateFromMessageRequest 从单条消息保存到私密文档请求
 type CreateFromMessageRequest struct {
 	MessageID      int64  `json:"message_id"`
-	ProjectID      int    `json:"project_id"`
+	ProjectID      int64  `json:"project_id"`
 	Title          string `json:"title,omitempty"`
 	ExpectedSHA256 string `json:"expected_sha256,omitempty"`
 }
 
 // CreateFreeformDocRequest 直接创建空白/自定义自由文档请求（上传附件即时注册事实源）
 type CreateFreeformDocRequest struct {
-	ProjectID       int    `json:"project_id"`
+	ProjectID       int64  `json:"project_id"`
 	Title           string `json:"title"`
 	ContentMarkdown string `json:"content_markdown"`
 	Space           string `json:"space,omitempty"` // 默认 'my_private'