feat: multi-account overview API with aggregated + per-account metrics (#132)

[Samfresh-ai]

/claim #132

Implements multi-account dashboard support with aggregated and per-account breakdowns.

What was added

• New endpoint: GET /dashboard/multi-account-overview
  • Optional query: account_keys=USD,INR,...
  • Returns:
    • aggregated totals (income, expenses, net flow, bills)
    • accounts[] per-account payloads
    • period, errors
• Reused existing dashboard summary calculation path through a shared builder
  • avoids duplicate logic and keeps consistency with /dashboard/summary

Account model used

This repo does not currently store explicit bank-account IDs in Expense, so account partitioning is derived from existing currency keys (e.g., USD/INR) for deterministic multi-account grouping using current schema.

Tests

• Added test: test_multi_account_overview_aggregates_and_returns_per_account
• Full relevant suite executed:
  • PYTHONPATH=. pytest -q tests/test_dashboard.py tests/test_auth.py tests/test_insights.py
  • Result: 9 passed

Files changed

• packages/backend/app/routes/dashboard.py
• packages/backend/tests/test_dashboard.py
• packages/backend/app/routes/auth.py
• packages/backend/app/services/cache.py
• packages/backend/tests/test_auth.py

This PR is focused on #132 multi-account overview functionality and includes passing test evidence.

[Samfresh-ai]

Scope cleanup pushed: removed unrelated auth/cache test changes. This PR now stays focused on multi-account overview only.

[Samfresh-ai]

Closing this to avoid noise and keep your queue clean. Thanks for the feedback.