feat(backend): Fix client-profile endpoint and add attachment support

- Fixed client-profile GET endpoint to prevent 500 errors
- Made ClientProfileResponse fields optional (id, created_at, updated_at)
- Returns empty profile data instead of creating DB entry on GET
- Added new attachment model and schemas for file handling
- Added file handler core utility for upload management

backend/app/api/schemas/attachment.py

@@ -0,0 +1,25 @@
+"""
+Attachment Schemas - ServiceManagerWeb
+"""
+from pydantic import BaseModel, ConfigDict, Field
+from datetime import datetime
+from typing import Optional
+import uuid
+
+
+class AttachmentResponse(BaseModel):
+    """Schema para respuesta de attachment"""
+    id: uuid.UUID
+    ticket_id: uuid.UUID
+    comment_id: Optional[uuid.UUID] = None
+    uploaded_by: uuid.UUID
+    filename: str
+    original_filename: str
+    mime_type: str
+    file_size: int
+    file_path: str
+    uploaded_by_name: Optional[str] = None
+    created_at: datetime
+    download_url: Optional[str] = None
+    
+    model_config = ConfigDict(from_attributes=True)