feat: Multi-account financial overview dashboard

[shaidshark]

Summary

This PR implements a multi-account financial overview dashboard as requested in #132.

Features Added

1. Account Model

   • Support for 6 account types: CHECKING, SAVINGS, CREDIT, INVESTMENT, CASH, OTHER
   • Fields: name, account_type, balance, currency, is_active
   • Full CRUD operations via /accounts endpoints

2. Dashboard Overview Endpoint (GET /dashboard/overview)

   • Net Worth: Total balance across all active accounts
   • Per-Account Breakdown: List of accounts with individual balances
   • Multi-Currency Support: Balance breakdown by currency
   • Recent Transactions: Last 10 transactions with account associations
   • Monthly Summary: Income and expenses for current month

3. Expense-Account Linking

   • Added �ccount_id field to Expense model
   • Transactions can be associated with specific accounts

4. Database Schema

   • Added �ccounts table with proper indexes
   • Added �ccount_id column to expenses table
   • PostgreSQL enum type for account_type

5. Tests

   • Comprehensive tests for Account CRUD operations
   • Tests for dashboard overview endpoint
   • Tests for all account types
   • Tests for multi-currency scenarios

6. Documentation

   • Updated README with multi-account feature documentation
   • Added API endpoint reference for accounts and overview

API Endpoints

Endpoint	Method	Description
/accounts/	GET	List all accounts (optional include_inactive)
/accounts/	POST	Create new account
/accounts/{id}	GET	Get specific account
/accounts/{id}	PUT	Update account
/accounts/{id}	DELETE	Soft delete (deactivate)
/accounts/{id}/hard	DELETE	Permanent deletion
/dashboard/overview	GET	Multi-account financial overview

Testing

All tests pass locally:

• Account CRUD tests
• Dashboard overview tests
• Multi-currency tests
• Authorization tests

Fixes #132