chore: add database migrations for due dates feature

Prepared SQL migrations for adding due_date support to todos.

Migrations:
- 003_add_due_date_to_todos.sql - Adds due_date column to todos table
- 004_add_due_date_to_global_todos.sql - Adds due_date column to global_todos table

Features:
- TIMESTAMPTZ column for timezone-aware dates
- Indexes on due_date for query performance
- Composite indexes on (completed, due_date) for filtering

Note: Backend and frontend implementation pending.
These migrations are ready to be applied when the feature is completed.

migrations/003_add_due_date_to_todos.sql

@@ -0,0 +1,11 @@
+-- Migration: Add due_date to todos table
+-- Description: Adds a due_date column to track task deadlines
+
+-- Add due_date column to todos table
+ALTER TABLE todos ADD COLUMN IF NOT EXISTS due_date TIMESTAMPTZ;
+
+-- Create index on due_date for faster queries
+CREATE INDEX IF NOT EXISTS idx_todos_due_date ON todos(due_date);
+
+-- Create index on combination of completed and due_date for efficient queries of pending tasks
+CREATE INDEX IF NOT EXISTS idx_todos_completed_due_date ON todos(completed, due_date);

migrations/004_add_due_date_to_global_todos.sql

@@ -0,0 +1,11 @@
+-- Migration: Add due_date to global_todos table
+-- Description: Adds a due_date column to track task deadlines for global todos
+
+-- Add due_date column to global_todos table
+ALTER TABLE global_todos ADD COLUMN IF NOT EXISTS due_date TIMESTAMPTZ;
+
+-- Create index on due_date for faster queries
+CREATE INDEX IF NOT EXISTS idx_global_todos_due_date ON global_todos(due_date);
+
+-- Create index on combination of completed and due_date for efficient queries of pending tasks
+CREATE INDEX IF NOT EXISTS idx_global_todos_completed_due_date ON global_todos(completed, due_date);