feat: add maintenance tree_type with db migration and tests

- Expand ck_trees_tree_type CHECK constraint to include 'maintenance'
- Add 'maintenance' to TreeType Literal in schemas
- Treat maintenance trees as procedural in can_publish_tree validation
- Alembic migration 0f1ca2af3647 drops and recreates the constraint
- Two integration tests: create and filter by tree_type=maintenance

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

backend/app/models/tree.py

@@ -29,7 +29,7 @@ class Tree(Base):
             name='ck_trees_status'
         ),
         CheckConstraint(
-            "tree_type IN ('troubleshooting', 'procedural')",
+            "tree_type IN ('troubleshooting', 'procedural', 'maintenance')",
             name='ck_trees_tree_type'
         ),
     )