feat: extend AI chat session with tree_id and archived_at

Add tree_id FK (CASCADE) for editor-embedded sessions and archived_at
timestamp column to ai_chat_sessions table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/ai_chat_session.py

@@ -86,3 +86,14 @@ class AIChatSession(Base):
         default=lambda: datetime.now(timezone.utc),
         onupdate=lambda: datetime.now(timezone.utc),
     )
+    # Editor-embedded session: links to a specific tree/flow
+    tree_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("trees.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True,
+    )
+    archived_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True),
+        nullable=True,
+    )