[FR] Budget tracking with category breakdown and summary

[candogruyol]

Built a budget tracking system on my fork:

• trip_budget table: per-category planned amounts (flights, accommodation, food, attractions, transport, shopping, misc)
• GET /api/trips/{id}/budget/summary returns: planned_total, actual_total (sum of item prices), breakdown_by_category, per_day costs
• Angular floating panel with progress bar (actual vs planned) and category list
• Categories validated via Literal types

Integrates with existing paid_by field. Also built a cost settlement endpoint (GET /api/trips/{id}/settlement) that calculates who owes whom for group trips.

Happy to PR if interested.