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/projection_service.py

@@ -8,7 +8,7 @@ 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.projection import project
 from app.engine.scenario import (PlainModifier, PlainPlanned, PlainRecurring,
                                  build_cashflows)
@@ -76,9 +76,7 @@ def run_projection(session: Session, scenario: Scenario, horizon_days: int,
                    start_date: date) -> ProjectionResult:
     end = start_date + timedelta(days=horizon_days)
     inputs = load_scenario_inputs(session, scenario)
-    schedules = [loan_schedule(Decimal(l.principal), Decimal(l.annual_rate_pct),
-                               l.term_months, l.payout_date, l.repayment_type)
-                 for l in inputs.loans]
+    schedules = [schedule_for(l) for l in inputs.loans]
     payouts = [(l.payout_date, Decimal(l.principal)) for l in inputs.loans]
     flows = build_cashflows(inputs.recurring, inputs.planned, schedules, payouts,
                             inputs.modifiers, start_date, end)