feat: goal-based savings tracking & milestones

[klawgulp-ship-it]

Summary

What

• Add SavingsGoal and SavingsMilestone SQLAlchemy models (backend/app/models/savings_goal.py)
• Add Pydantic request/response schemas (backend/app/schemas/savings_goal.py)
• Implement REST API routes for full CRUD on goals and milestones (backend/app/routes/savings.py)
• Add typed frontend API client (app/src/api/savings.ts)
• Add unit/integration tests covering all endpoints (backend/tests/test_savings.py)
• Add feature documentation (docs/savings-goals.md)

Why

• Fixes Goal-based savings tracking & milestones #133 — users can now create savings goals, track progress against a target amount, and define milestones to celebrate along the way

Changes

• Define SavingsGoal and SavingsMilestone SQLAlchemy models
• Add Pydantic schemas for SavingsGoal and Milestone
• Implement REST API routes for savings goals and milestones CRUD
• Add unit/integration tests for savings goals and milestones routes
• Implement typed frontend API client for savings goals and milestones
• Add documentation for goal-based savings tracking feature

Testing

• Verified the changes align with the issue requirements
• Kept modifications minimal and surgical to reduce review burden

Closes #133