feat: multi-account financial overview dashboard (#132)

[sinatragianpaolo-oc]

Summary

Implements Issue #132: Multi-account financial overview — view multiple financial accounts in one place.

Features

Account types supported

BANK, CREDIT, CASH, INVESTMENT, WALLET, OTHER

Endpoints

Method	Path	Description
GET	/accounts	List active accounts
POST	/accounts	Create account
GET	/accounts/<id>	Get account detail
PATCH	/accounts/<id>	Update account
DELETE	/accounts/<id>	Soft-delete (deactivate)
GET	/accounts/overview	Aggregate view: balance per account + net worth

Overview response

{
  "accounts": [
    {"name": "Main Bank", "balance": 8000.0, "income": 0, "expenses": 2000.0, ...}
  ],
  "summary": {
    "total_assets": 10500.0,
    "total_liabilities": 2000.0,
    "net_worth": 8500.0,
    "unassigned_income": 0.0,
    "unassigned_expenses": 300.0,
    "account_count": 3
  }
}

Backward compatibility

expenses.account_id is nullable — existing expenses continue to work unchanged. Unassigned expenses appear in summary.unassigned_expenses.

Net worth calculation

• Assets = sum of BANK + CASH + INVESTMENT + WALLET + OTHER balances
• Liabilities = sum of CREDIT balances
• Net worth = Assets − Liabilities

Files changed

File	Description
app/models.py	Account + AccountType + Expense.account_id
app/routes/accounts.py	All account endpoints
app/routes/__init__.py	Register accounts blueprint
app/db/migrations/005_accounts.sql	DB migration
tests/test_accounts.py	27 tests

Acceptance Criteria

• Production-ready implementation
• Includes tests (27 tests)
• Documentation updated

Closes #132