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/session.py

@@ -0,0 +1,46 @@
+import uuid
+from datetime import datetime
+from typing import Optional, Any
+from sqlalchemy import String, DateTime, ForeignKey, Boolean
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from app.core.database import Base
+
+
+class Session(Base):
+    __tablename__ = "sessions"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    tree_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id"),
+        nullable=False,
+        index=True
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=False,
+        index=True
+    )
+    tree_snapshot: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
+    path_taken: Mapped[list[str]] = mapped_column(JSONB, nullable=False, default=list)
+    decisions: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list)
+    started_at: Mapped[datetime] = mapped_column(
+        DateTime,
+        default=datetime.utcnow,
+        index=True
+    )
+    completed_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True, index=True)
+    ticket_number: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
+    client_name: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    exported: Mapped[bool] = mapped_column(Boolean, default=False)
+
+    # Relationships
+    tree: Mapped["Tree"] = relationship("Tree", back_populates="sessions")
+    user: Mapped["User"] = relationship("User", back_populates="sessions")
+    attachments: Mapped[list["Attachment"]] = relationship("Attachment", back_populates="session")