feat: goal-based savings tracking & milestones

[cyrillical00]

Closes #133

Summary

• Adds SavingsGoal SQLAlchemy model with target_amount, current_amount, currency, deadline fields
• REST API under /savings: list, create, get, update, delete goals; contribute amounts
• Status computation (completed/on-track/ahead/behind) based on progress relative to deadline
• Monthly target calculation based on remaining amount and months until deadline
• Goals default to the authenticated user's preferred currency
• React frontend: Budgets.tsx loads real savings goals from the API (with loading/empty states)
• Frontend API client at app/src/api/savings.ts
• 17 passing backend tests using fakeredis for Redis isolation

Test plan

• cd packages/backend && python -m pytest tests/test_savings.py -v → 17 passed
• Create a savings goal via POST /savings and verify status field
• Contribute to a goal and verify status flips to completed when current >= target
• Verify goals are isolated between users
• Open Budgets page in browser — goals panel loads from API