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/tree.py

@@ -0,0 +1,51 @@
+import uuid
+from datetime import datetime
+from typing import Optional, Any
+from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, Index
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from app.core.database import Base
+
+
+class Tree(Base):
+    __tablename__ = "trees"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    description: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    category: Mapped[Optional[str]] = mapped_column(String(100), nullable=True, index=True)
+    tree_structure: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
+    author_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=True
+    )
+    team_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("teams.id"),
+        nullable=True,
+        index=True
+    )
+    is_active: Mapped[bool] = mapped_column(Boolean, default=True)
+    version: Mapped[int] = mapped_column(Integer, default=1)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime,
+        default=datetime.utcnow
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime,
+        default=datetime.utcnow,
+        onupdate=datetime.utcnow
+    )
+    usage_count: Mapped[int] = mapped_column(Integer, default=0)
+
+    # Relationships
+    author: Mapped[Optional["User"]] = relationship("User", back_populates="trees")
+    team: Mapped[Optional["Team"]] = relationship("Team", back_populates="trees")
+    sessions: Mapped[list["Session"]] = relationship("Session", back_populates="tree")
+
+    # Full-text search index will be created in migration