- replace Database with db instance
- use schema imports for tables
- use db.select, db.insert, onConflictDoUpdate
- remove manual connection handling and console logs
- improve type safety and maintainability
refactor(students): migrate to Drizzle ORM and async queries
Replace raw sqlite queries with Drizzle ORM. Remove the connect helper and use the
shared db instance and schema definitions. Convert getItself, getAll and
addStudents to async functions, use eq and lt helpers, and simplify promotion
handling. This improves type safety, maintainability, and allows non‑blocking
database access.