feat: implement tree sharing, draft trees, and session-to-tree conversion (Issues #16, #25, #17)

Backend features:
- Tree sharing via secure tokens with expiration (Issue #16)
- Draft tree status with conditional validation (Issue #25)
- Save session as custom tree with fork tracking (Issue #17)
- Tree validation system for publish requirements
- Session-to-tree conversion preserving custom steps

Database migrations:
- 024: Tree sharing (tree_shares table, visibility field)
- 025: Tree status field (draft/published)
- 25b: Merge migration for indexes

New endpoints:
- POST /api/v1/trees/{id}/share - Generate share token
- GET /api/v1/shared/{token} - Public tree access
- POST /api/v1/trees/{id}/can-publish - Validate tree
- POST /api/v1/sessions/{id}/save-as-tree - Convert session

Test coverage:
- 20 tests for draft trees functionality
- 14 tests for session-to-tree conversion
- 15 tests for tree sharing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/models/tree.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime, timezone
 from typing import Optional, Any, TYPE_CHECKING
-from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, Index
+from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, Index, CheckConstraint
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.core.database import Base
@@ -14,10 +14,21 @@ if TYPE_CHECKING:
     from app.models.category import TreeCategory
     from app.models.tag import TreeTag
     from app.models.folder import UserFolder
+    from app.models.tree_share import TreeShare
 
 
 class Tree(Base):
     __tablename__ = "trees"
+    __table_args__ = (
+        CheckConstraint(
+            "visibility IN ('private', 'team', 'link', 'public')",
+            name='ck_trees_visibility'
+        ),
+        CheckConstraint(
+            "status IN ('draft', 'published')",
+            name='ck_trees_status'
+        ),
+    )
 
     id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
@@ -57,6 +68,20 @@ class Tree(Base):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
     is_public: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
     is_default: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
+    visibility: Mapped[str] = mapped_column(
+        String(20),
+        nullable=False,
+        default='team',
+        index=True,
+        comment="Visibility level: private (author only), team (account members), link (share token), public (all users)"
+    )
+    status: Mapped[str] = mapped_column(
+        String(20),
+        nullable=False,
+        default='published',
+        index=True,
+        comment="Status: draft (work in progress) or published (validated and available)"
+    )
     deleted_at: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True),
         nullable=True,
@@ -136,6 +161,11 @@ class Tree(Base):
         foreign_keys=[root_tree_id]
     )
     sessions: Mapped[list["Session"]] = relationship("Session", back_populates="tree")
+    shares: Mapped[list["TreeShare"]] = relationship(
+        "TreeShare",
+        back_populates="tree",
+        cascade="all, delete-orphan"
+    )
 
     # New organization relationships
     category_rel: Mapped[Optional["TreeCategory"]] = relationship("TreeCategory", back_populates="trees")