feat: initial backend setup with Bun, Hono, and Drizzle ORM

- Added documentation for architecture and code plan
- Implemented database schema for users, expenses, settlements, and transactions
- Set up double-entry bookkeeping system with multi-currency support
- Configured Hono web framework with middleware
- Integrated Drizzle ORM for database operations

backend/drizzle.config.ts

@@ -0,0 +1,12 @@
+import type { Config } from 'drizzle-kit';
+
+export default {
+  schema: './src/db/schema/*',
+  out: './src/db/migrations',
+  driver: 'pg',
+  dbCredentials: {
+    connectionString: process.env.DATABASE_URL || 'postgres://postgres:postgres@localhost:5432/splitwise',
+  },
+  verbose: true,
+  strict: true,
+} satisfies Config;