feat: Implement SQLite Database Persistence with GORM

[frye]

Summary

Replaces the in-memory user slice with a real SQLite database using GORM, ensuring data persistence across application restarts.

Closes #15

Changes

New files

• database/database.go — InitDB (opens SQLite + auto-migrate) and SeedDB (populates default users if empty)
• database/testhelper.go — Shared in-memory SQLite helper for tests
• database/database_test.go — Tests for InitDB, SeedDB, and seed idempotency
• controllers/user_controller_test.go — 12 tests covering all CRUD endpoints and edge cases

Modified files

• models/user.go — Added GORM primaryKey tag, changed ID from string to uint, added CreateUserRequest/UpdateUserRequest DTOs
• controllers/user_controller.go — Refactored to struct-based UserController with DB injection; all handlers now use GORM queries; implemented DeleteUser
• api/routes.go — Accepts *gorm.DB, wires up controller methods, added DELETE /:id route
• main.go — Initializes database, seeds data, passes DB to router
• go.mod / go.sum — Added gorm.io/gorm and gorm.io/driver/sqlite

Testing

15 tests pass covering:

• Database initialization and migration
• Seed idempotency
• Full CRUD (GET all, GET by ID, POST, PUT, DELETE)
• Error cases (invalid ID → 400, not found → 404, missing fields → 400)