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/schemas/tree.py

@@ -7,7 +7,7 @@ import re
 
 # --- Tree Type ---
 
-TreeType = Literal['troubleshooting', 'procedural']
+TreeType = Literal['troubleshooting', 'procedural', 'maintenance']
 
 # --- Intake Form Schemas ---