feat(backend): PostgreSQL connection with Drizzle ORM + base schema

[codebestia]

Summary

• Installed drizzle-orm and postgres driver; added drizzle-kit as a dev dependency
• Created src/db/index.ts — initializes a typed Drizzle client from DATABASE_URL
• Defined base schema in src/db/schema.ts — users table (id, username, avatar_url, timestamps) and wallets table (id, user_id FK → users, address, is_primary, created_at)
• Added drizzle.config.ts and db:generate, db:migrate, db:push, db:studio scripts to package.json
• Generated initial SQL migration: drizzle/0000_init.sql
• Upgraded /health endpoint to run SELECT 1 and report DB connectivity status