feat: Multi-account financial overview dashboard

[chengyixu]

/claim #132

Summary

Implements a comprehensive multi-account financial overview dashboard that allows users to link and manage multiple bank/financial accounts with a unified view across all accounts.

Backend

• FinancialAccount model with 7 account types: CHECKING, SAVINGS, CREDIT_CARD, INVESTMENT, LOAN, CASH, OTHER
• Full CRUD API at /accounts with soft-delete (deactivation) support
• Aggregated overview at /accounts/overview computing:
  • Net worth (total assets minus total liabilities)
  • Total assets and total liabilities
  • Per-account monthly income, expenses, net flow, and transaction count
  • Combined recent transactions across all accounts
• Per-account transactions at /accounts/{id}/transactions with pagination
• Expenses linked to accounts via new account_id foreign key
• SQL schema migration for financial_accounts table and expenses.account_id
• OpenAPI spec updated with all new endpoints and schemas

Frontend

• New /accounts page with:
  • Net Worth, Total Assets, Total Liabilities, Monthly Net summary cards
  • Accounts grouped by type with per-account income/expense stats
  • Create/edit account dialog (name, type, institution, balance, currency)
  • Soft-delete accounts
  • Cross-account recent transactions sidebar
  • Month selector for period filtering
• Navigation bar updated with "Accounts" link
• Route registered in App.tsx

Tests

• 18 new backend pytest tests covering:
  • CRUD operations (create, list, get, update, soft-delete)
  • Validation (name required, invalid type)
  • Overview aggregation (empty, with accounts, with transactions, month filter)
  • Per-account transactions
  • Edge cases (not found, all account types)
• 8 frontend integration tests covering:
  • Page rendering and summary cards
  • Account grouping by type
  • Recent transactions display
  • Empty state and error handling
• All 40 backend tests passing (18 new + 22 existing, zero regressions)

Documentation

• README updated with accounts endpoints, schema, and UI plan
• OpenAPI spec includes FinancialAccount, AccountsOverview schemas

Test plan

• All 40 backend tests pass (pytest tests/ -v)
• Frontend integration tests cover core Accounts page functionality
• Zero regressions on existing test suite
• Schema backward-compatible (new table + nullable column addition)
• Manual verification: create accounts of different types, check overview aggregation
• Manual verification: link transactions to accounts, verify per-account breakdown