Track Freistellungsauftrag utilization

[rappm]

Goal

Track exemption order usage and alert users as the limit is approached.

Source

• FR-T2 in deliverables/PROBLEM_STATEMENT.md

Change

• Model the user's Freistellungsauftrag limit and consumed amount.
• Calculate utilization from relevant income events.
• Alert users near the configured threshold.

Done When

• A user can view Freistellungsauftrag utilization.
• The system alerts when the exemption limit is approached.
• Utilization updates when taxable events are imported or calculated.