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

docs/features.md

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+# Features
+
+This document outlines the core features for the MVP of the Splitwise‑style application, focusing on essential expense‑splitting functionality without groups or advanced integrations.
+
+## 1. User & Account
+
+* **Sign Up / Sign In**
+
+  * Email/password authentication (OAuth support in later releases)
+* **Profile**
+
+  * Display name, email, profile photo
+  * Default currency, locale, time zone
+* **Contacts**
+
+  * Add/remove contacts by email or invite link
+  * Accept or decline contact requests
+
+## 2. Expense Tracking
+
+* **Add Expense**
+
+  * Title, total amount, date/time
+  * Payer (single user)
+  * Participants & split rules (equal or custom share)
+  * Optional memo/notes
+  * Upload receipt image (optional)
+* **Edit & Delete**
+
+  * Modify any field or remove an expense
+
+## 3. Balances & Settlements
+
+* **Real‑time Balances**
+
+  * “You owe / you are owed” per‑contact and overall summaries
+* **Settle Up**
+
+  * Record a payment against one or more open balances
+  * Mark balances as paid to zero them out
+
+## 4. Activity Feed / Change Log
+
+* Chronological feed of who added, edited, or settled each expense
+* Timestamps and links back to the full expense details
+
+## 5. Notifications & Reminders
+
+* **In‑app / Push Alerts**
+
+  * New expense involving you
+  * Someone settles up with you
+* **Email Summaries** (optional for v1)
+
+  * Daily or weekly digest of outstanding balances
+
+## 6. Settings & Preferences
+
+* **Currency & Formatting**
+
+  * Set default currency (single‑currency MVP)
+  * Date and number formats (e.g. DD/MM/YYYY)
+* **Notification Controls**
+
+  * Toggle which alerts you receive
+
+---
+
+*End of MVP Feature List for initial release.*
+