feat: add backend support for hierarchical subtasks

- Create migration 005 to add parent_id and level columns to todos tables
- Fix migration 003 to use correct table name (note_todos instead of todos)
- Update POST /api/notes/:id/todos to accept parent_id parameter
- Update POST /api/todos to accept parent_id for global todos
- Add automatic level calculation based on parent depth
- Update GET endpoints to return parent_id and level fields
- Add validation to ensure parent todo exists before creating subtask
- Support CASCADE DELETE for subtasks when parent is deleted

migrations/005_add_subtasks_support.sql

@@ -0,0 +1,16 @@
+-- Migration: Add subtasks support to todos tables
+-- Description: Adds parent_id column to enable hierarchical task structure
+
+-- Add parent_id column to note_todos table (note todos)
+ALTER TABLE note_todos ADD COLUMN IF NOT EXISTS parent_id INTEGER REFERENCES note_todos(id) ON DELETE CASCADE;
+
+-- Add parent_id column to global_todos table
+ALTER TABLE global_todos ADD COLUMN IF NOT EXISTS parent_id INTEGER REFERENCES global_todos(id) ON DELETE CASCADE;
+
+-- Create indexes for faster subtask queries
+CREATE INDEX IF NOT EXISTS idx_note_todos_parent_id ON note_todos(parent_id);
+CREATE INDEX IF NOT EXISTS idx_global_todos_parent_id ON global_todos(parent_id);
+
+-- Add level column to track depth (optional, for UI optimization)
+ALTER TABLE note_todos ADD COLUMN IF NOT EXISTS level INTEGER DEFAULT 0;
+ALTER TABLE global_todos ADD COLUMN IF NOT EXISTS level INTEGER DEFAULT 0;