refactor: Tilgungsplan-Helfer schedule_for, Kredit-Laufzeit 1–1200 Monate (A14-T5)

- Neuer schedule_for() in app/engine/loans.py: vereinheitlicht Tilgungsplan-Erstellung
- Laufzeit-Validierung in LoanIn und LoanPatch: 1–1200 Monate
- Aufrufstellen aktualisiert (routers/planning.py, services/breakdown.py,
  services/projection_service.py, services/planning_chart.py)
- Keine direkten loan_schedule(Decimal(...)) Aufrufe außerhalb engine/loans.py mehr
- Tests für schedule_for und Laufzeit-Grenzen (1, 1200, sowie 0/-5/1201 Ablehnung)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

finance/app/services/planning_chart.py

@@ -15,7 +15,7 @@ from sqlalchemy.orm import Session
 
 from app.config import get_settings
 from app.db import advisory_xact_lock
-from app.engine.loans import loan_schedule
+from app.engine.loans import schedule_for
 from app.engine.recurrence import occurrences
 from app.models.tables import (Category, Loan, PlannedItem, PlanningMonthly,
                                RecurringItem)
@@ -46,9 +46,7 @@ def planning_monthly_totals(session: Session, today: date,
         if window_start <= p.due <= window_end:
             totals[(p.due.replace(day=1), category(p.category_id))] += Decimal(p.amount)
     for loan in session.execute(select(Loan)).scalars():
-        for inst in loan_schedule(Decimal(loan.principal), Decimal(loan.annual_rate_pct),
-                                  loan.term_months, loan.payout_date,
-                                  loan.repayment_type):
+        for inst in schedule_for(loan):
             if window_start <= inst.due <= window_end:
                 # payment ist bereits negativ (Abfluss), wie in build_cashflows.
                 totals[(inst.due.replace(day=1), KREDITRATEN)] += inst.payment