Generate and commit a baseline migration

There's no migration history at all right now. drizzle.config.ts declares out: "./drizzle" but the folder doesn't exist, and the two hand-written SQL files in backend/migrations/ live outside any system that tracks whether they ran.

• Run drizzle-kit generate against schema.ts to produce the baseline migration + journal (drizzle/0000_*.sql and meta/_journal.json)
• Commit the drizzle/ folder, it's the source of truth from here on
• Archive backend/migrations/*.sql, their changes are already part of schema.ts so the baseline covers them
• Make sure the folder isn't gitignored