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/core/tree_validation.py

@@ -224,14 +224,14 @@ def can_publish_tree(
         errors.append({"field": "name", "message": "Tree must have a name to be published"})
 
     # Validate structure based on tree type
-    if tree_type == "procedural":
+    if tree_type in ("procedural", "maintenance"):
         structure_valid, structure_errors = validate_procedural_structure(tree_structure)
     else:
         structure_valid, structure_errors = validate_tree_structure(tree_structure)
     errors.extend(structure_errors)
 
     # Validate intake form if present (procedural only)
-    if intake_form and tree_type == "procedural":
+    if intake_form and tree_type in ("procedural", "maintenance"):
         form_valid, form_errors = _validate_intake_form(intake_form)
         errors.extend(form_errors)