Initial commit: Backend API Phase 1a complete

- FastAPI backend with JWT auth
- PostgreSQL database schema
- Trees and Sessions CRUD APIs
- Export functionality (Markdown, Text, HTML)
- Docker setup for local development
- Alembic migrations

backend/app/models/attachment.py

@@ -0,0 +1,34 @@
+import uuid
+from datetime import datetime
+from typing import Optional
+from sqlalchemy import String, Integer, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.core.database import Base
+
+
+class Attachment(Base):
+    __tablename__ = "attachments"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    session_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("sessions.id"),
+        nullable=False
+    )
+    node_id: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
+    file_name: Mapped[str] = mapped_column(String(255), nullable=False)
+    file_type: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
+    file_size: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    storage_path: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
+    uploaded_at: Mapped[datetime] = mapped_column(
+        DateTime,
+        default=datetime.utcnow
+    )
+
+    # Relationships
+    session: Mapped["Session"] = relationship("Session", back_populates="attachments")