feat(backend): add Knex database migration system

[KarenZita01]

Summary

Closes #91

Implements a database migration system using Knex.js, ready for when a persistent database is introduced.

Checklist

• Choose migration tool — Knex.js (native JS/ESM, no extra runtime, supports SQLite + PostgreSQL)
• Create initial migration — create_assets_table with core columns and rollback
• Add migration run commands to package.json
• Document how to create and run migrations

Changes

File	Description
backend/knexfile.js	Knex config — SQLite for dev/test, PostgreSQL for production
backend/migrations/20260628000000_create_assets_table.js	Initial migration: assets table
backend/docs/migrations.md	Full usage documentation
backend/package.json	Added migrate, migrate:rollback, migrate:make, migrate:status scripts; added knex, better-sqlite3, pg deps
backend/.env.example	Added DATABASE_URL documentation

Migration commands

npm run migrate            # apply pending migrations
npm run migrate:rollback   # roll back last batch
npm run migrate:make -- <name>  # scaffold a new migration
npm run migrate:status     # list applied / pending

Migration state tracking

Knex automatically creates a knex_migrations table on first run to record which migrations have been applied and in which batch — enabling safe rollbacks.

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