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/.gitignore

@@ -0,0 +1,34 @@
+# Logs
+logs
+*.log
+npm-debug.log*
+
+# Dependency directories
+node_modules/
+
+# TypeScript cache
+*.tsbuildinfo
+
+# Environment variables
+.env
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+# Build output
+dist/
+build/
+
+# Coverage directory
+coverage/
+
+# Mac files
+.DS_Store
+
+# Temporary files
+*.tmp
+*.temp
+
+# Bun
+.bun