feat: add savings goals and milestones

[helh1723-lang]

Summary

• Adds authenticated savings goal CRUD endpoints with progress percentage, remaining balance, milestone state, and audit logging.
• Adds Postgres schema/init compatibility for savings goals and milestones.
• Adds a Savings Goals UI page, API client, navigation entry, backend tests, README notes, and OpenAPI docs.

Fixes #133

Validation

• python -m py_compile packages\backend\app\models.py packages\backend\app\routes\savings_goals.py packages\backend\app\routes_init_.py packages\backend\app_init_.py packages\backend\tests\test_savings_goals.py
• git diff --check

Notes

• Full backend pytest could not be run locally because pytest is not installed in this environment.
• Frontend build could not be run locally because node_modules/tsc is not installed in this environment.

[helh1723-lang]

/claim #133