feat: trigger step library sync on tree publish and deactivate on delete

- Call sync_steps_from_tree in update_tree whenever the tree is published
  (status transitions to 'published' or is already published and structure changes)
- Call deactivate_synced_steps_for_tree in delete_tree before db.commit()
  so the FK SET NULL does not nullify source_tree_id before the WHERE clause runs
- Fix ::jsonb cast syntax in step_sync.py (asyncpg rejects :: operator in text()
  queries; replaced with CAST(:content AS jsonb))
- Add UniqueConstraint('source_tree_id','source_node_id') to StepLibrary model
  so Base.metadata.create_all (used by tests) creates the constraint that the
  ON CONFLICT clause in sync_steps_from_tree depends on

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

backend/app/api/endpoints/trees.py

@@ -28,6 +28,7 @@ from app.core.subscriptions import check_tree_limit
 from app.core.audit import log_audit
 from app.core.config import settings
 from app.core.tree_validation import can_publish_tree
+from app.core.step_sync import sync_steps_from_tree, deactivate_synced_steps_for_tree
 
 router = APIRouter(prefix="/trees", tags=["trees"])
 
@@ -640,6 +641,21 @@ async def update_tree(
     if "tree_structure" in update_data:
         tree.version += 1
 
+    # Sync steps to step library when publishing
+    if update_data.get("status") == 'published' or tree.status == 'published':
+        _structure = update_data.get("tree_structure", tree.tree_structure)
+        _type = update_data.get("tree_type", tree.tree_type)
+        _is_public = update_data.get("is_public", tree.is_public)
+        await sync_steps_from_tree(
+            db=db,
+            tree_id=tree.id,
+            tree_type=_type,
+            tree_structure=_structure,
+            author_id=tree.author_id,
+            account_id=tree.account_id,
+            is_public=_is_public,
+        )
+
     # Handle tags replacement
     if tags_data is not None:
         from app.models.tag import tree_tag_assignments
@@ -753,6 +769,10 @@ async def delete_tree(
         tree_tag_assignments.delete().where(tree_tag_assignments.c.tree_id == tree.id)
     )
 
+    # Deactivate any synced step library entries before deletion
+    # (must happen before db.delete/commit — FK SET NULL would lose the reference)
+    await deactivate_synced_steps_for_tree(db, tree.id)
+
     await log_audit(db, current_user.id, "tree.delete", "tree", tree.id,
                     {"tree_name": tree.name})
     await db.commit()