Remove hardcoded IDs from SQL examples in databases week1

[urbanogilson]

Why

Hardcoded IDs teach students a fragile pattern that breaks when rows are inserted in a different order. Using subqueries on stable natural keys is the correct habit to build from the start.

Summary

• Replace hardcoded user_id/task_id integers in user_task inserts with subqueries on user.email and task.title
• Replace hardcoded status_id integers with subqueries on status.name
• Add PRAGMA foreign_keys = ON to tasks-solution.sql and session-plan.md to fix silent ON DELETE CASCADE failures in SQLite
• Add inline notes explaining the DEFAULT 1 and PRAGMA SQLite limitations