feat(lesson25): DB queries + library_users model/repo + DB loader wiring

[JoyBrown252]

What I changed

• Added SQL script files (counts by type, sum pages checked out, guests with checkouts)
• Created library_users table + seed data
• Implemented LibraryUserModel and LibraryUserRepository
• Implemented LibraryDbDataLoader#loadData() and wired users into models.LibraryDataModel

How to test

1. Seed users (already done):
   cd lesson_25/db/db_app/src/main/resources/sqlite
   sqlite3 library.db ".read joy_add_library_users.sql"
   sqlite3 library.db "SELECT COUNT(*) FROM library_users;" # expect 3

2. Run the app:
   ./lesson_25/db/gradlew -p lesson_25/db :db_app:run --console=plain

   App starts; no SQL errors; CLI menu appears.

3. (Optional) Execute queries:
   sqlite3 library.db ".read ../queries/joy_counts_by_type.sql"
   sqlite3 library.db ".read ../queries/joy_sum_pages_checked_out.sql"
   sqlite3 library.db ".read ../queries/joy_guests_with_checkouts.sql"