feat(model,service): add folder_path and user_id to FileResponse, add user_id filter to ListFiles

- FileResponse gains folder_path ("/" for root) and user_id fields

- folder_id no longer uses omitempty, root files return null

- ListFiles accepts optional userID parameter for filtering by owner

- New buildFileResponse helper populates folder_path from FolderStore

- New GetFileResponse method wraps GetFileMetadata + buildFileResponse

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/model/file.go

@@ -130,14 +130,16 @@ type UploadSessionResponse struct {
 
 // FileResponse is the DTO for a file in API responses.
 type FileResponse struct {
-	ID        int64     `json:"id"`
-	Name      string    `json:"name"`
-	FolderID  *int64    `json:"folder_id,omitempty"`
-	Size      int64     `json:"size"`
-	MimeType  string    `json:"mime_type"`
-	SHA256    string    `json:"sha256"`
-	CreatedAt time.Time `json:"created_at"`
-	UpdatedAt time.Time `json:"updated_at"`
+	ID         int64     `json:"id"`
+	Name       string    `json:"name"`
+	FolderID   *int64    `json:"folder_id"`
+	FolderPath *string   `json:"folder_path"`
+	UserID     *int64    `json:"user_id"`
+	Size       int64     `json:"size"`
+	MimeType   string    `json:"mime_type"`
+	SHA256     string    `json:"sha256"`
+	CreatedAt  time.Time `json:"created_at"`
+	UpdatedAt  time.Time `json:"updated_at"`
 }
 
 // FolderResponse is the DTO for a folder in API responses.