fix: handle foreign key constraint when deleting tasks with selected_task_id refs

Goals table has selected_task_id referencing tasks(id) without
ON DELETE SET NULL. When deleting a task, first clear any
selected_task_id references in goals to prevent FK violation.

Also update schema for future databases.

database.py

@@ -190,6 +190,7 @@ def update_task(task_id, **kwargs):
 def delete_task(task_id):
     conn = get_connection()
     try:
+        conn.execute('UPDATE goals SET "selected_task_id" = NULL WHERE "selected_task_id" = ?', (task_id,))
         conn.execute("DELETE FROM tasks WHERE id = ?", (task_id,))
         conn.commit()
     finally:

schema.py

@@ -22,7 +22,7 @@ CREATE TABLE IF NOT EXISTS goals (
     activated INTEGER NOT NULL DEFAULT 1,
     selected_task_id INTEGER,
     FOREIGN KEY (user_id) REFERENCES users(id),
-    FOREIGN KEY (selected_task_id) REFERENCES tasks(id)
+    FOREIGN KEY (selected_task_id) REFERENCES tasks(id) ON DELETE SET NULL
 )
 """