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/models/attachment.py

@@ -0,0 +1,62 @@
+"""
+Attachment Model - ServiceManagerWeb
+"""
+from sqlalchemy import String, ForeignKey, Integer, DateTime, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from typing import Optional, TYPE_CHECKING
+from datetime import datetime
+import uuid
+
+from app.core.database import Base
+
+if TYPE_CHECKING:
+    from app.models.ticket import Ticket
+    from app.models.comment import TicketComment
+    from app.models.user import User
+
+
+class TicketAttachment(Base):
+    """Modelo de archivos adjuntos en tickets"""
+    __tablename__ = "ticket_attachments"
+    
+    # Sobrescribir campos heredados de Base para que coincidan con la tabla real
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    # Esta tabla NO tiene updated_at, así que lo excluimos del mapping
+    
+    ticket_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("tickets.id", ondelete="CASCADE"),
+        nullable=False
+    )
+    
+    comment_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("ticket_comments.id", ondelete="CASCADE"),
+        nullable=True
+    )
+    
+    uploaded_by: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=False
+    )
+    
+    filename: Mapped[str] = mapped_column(String(255), nullable=False)
+    original_filename: Mapped[str] = mapped_column(String(255), nullable=False)
+    mime_type: Mapped[str] = mapped_column(String(100), nullable=False)
+    file_size: Mapped[int] = mapped_column(Integer, nullable=False)
+    file_path: Mapped[str] = mapped_column(String(500), nullable=False)
+    
+    md5_hash: Mapped[Optional[str]] = mapped_column(String(32), nullable=True)
+    sha256_hash: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
+    
+    ticket: Mapped["Ticket"] = relationship("Ticket", back_populates="attachments")
+    comment: Mapped[Optional["TicketComment"]] = relationship("TicketComment", back_populates="attachments")
+    uploaded_by_user: Mapped["User"] = relationship("User")
+    
+    # Excluir updated_at del mapping ya que la tabla no lo tiene
+    __mapper_args__ = {
+        "exclude_properties": ["updated_at"]
+    }