feat: Goal-based savings tracking & milestones

[gent33112-wq]

Summary

Allow users to track savings goals and milestones.

Changes

1. SavingsGoal model - New database model with fields: name, target_amount, current_amount, currency, deadline, completed
2. REST API - Full CRUD endpoints:
   • GET /api/savings-goals - List all goals
   • POST /api/savings-goals - Create goal
   • PATCH /api/savings-goals/:id - Update progress
   • DELETE /api/savings-goals/:id - Delete goal
3. Schema - Added savings_goals table

Usage

# Create a goal
curl -X POST /api/savings-goals -d {"name":"Vacation","target_amount":1000}

# Update progress
curl -X PATCH /api/savings-goals/1 -d {"current_amount":500}

# List goals
curl /api/savings-goals

/claim #133