fix: Add default empty string to tasks.assigned_to column

src/database/schema.sql

@@ -11,7 +11,7 @@ CREATE TABLE IF NOT EXISTS groups (
 CREATE TABLE IF NOT EXISTS tasks (
   id INTEGER PRIMARY KEY AUTOINCREMENT,
   description TEXT NOT NULL,
-  assigned_to TEXT NOT NULL, -- WhatsApp phone number
+  assigned_to TEXT NOT NULL DEFAULT '', -- WhatsApp phone number (empty string if unassigned)
   due_date TEXT, -- ISO 8601 date string (e.g., "2023-10-31")
   completed BOOLEAN NOT NULL DEFAULT FALSE,
   created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,