feat: Multi-account financial overview dashboard

[inventelpk-cell]

Summary

Adds multi-account support to FinMind, allowing users to manage multiple financial accounts (checking, savings, credit, investment) and view aggregated financial data across all accounts in a unified dashboard.

Closes #132

Changes

New Account Model

• Account model with fields: id, user_id, name, account_type (CHECKING/SAVINGS/CREDIT/INVESTMENT), balance, currency, is_default, is_active (soft delete), created_at, updated_at
• AccountType enum for type safety
• Nullable account_id FK added to Expense model for backward compatibility

API Endpoints

• POST /accounts — Create account (first account auto-defaults)
• GET /accounts — List user's active accounts (cached, 5min TTL)
• GET /accounts/<id> — Account detail
• PATCH /accounts/<id> — Update account fields
• DELETE /accounts/<id> — Soft delete with automatic default promotion
• GET /accounts/<id>/summary?month=YYYY-MM — Per-account expense summary with category breakdown
• GET /dashboard/overview?month=YYYY-MM — Aggregated view across all accounts (total balance, per-account income/expenses/net flow)
• GET /dashboard/summary?account_id=N — Existing dashboard now supports optional account filtering
• GET /expenses?account_id=N — Existing expense list now supports account filtering

Database

• Idempotent migration: packages/backend/app/db/migrations/001_add_accounts.sql
• Schema compatibility patch in __init__.py for existing deployments
• Updated schema.sql with accounts table and expense FK

Caching & Invalidation

• New Redis cache keys: user:{id}:accounts, user:{id}:account_summary:{acct}:{ym}, user:{id}:dashboard_overview:{ym}
• Account mutations invalidate all related caches including dashboard summaries

Tests

• 8 pytest tests covering: full CRUD lifecycle, input validation, default account logic, per-account summaries, dashboard overview aggregation, dashboard filtering, expense filtering, 404 handling

Backward Compatibility

• account_id on expenses is nullable — existing expenses without accounts continue to work
• Existing /dashboard/summary endpoint behavior is unchanged when account_id param is omitted
• Migration is idempotent (IF NOT EXISTS / IF NOT EXISTS)

Test Plan

• Run pytest tests/test_accounts.py — 8 new tests
• Run pytest tests/ — all 30 tests pass (existing tests unaffected)
• Verify accounts CRUD via API manually
• Verify /dashboard/overview aggregates correctly across accounts
• Verify /dashboard/summary?account_id=N filters correctly
• Verify soft delete promotes next account to default