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/routers/planning.py

@@ -9,7 +9,7 @@ from sqlalchemy.orm import Session
 
 from app.auth import require_auth
 from app.db import get_session
-from app.engine.loans import loan_schedule
+from app.engine.loans import schedule_for
 from app.models.tables import (Category, Loan, PlannedItem, RecurringItem,
                                ScenarioLoan)
 from app.services.double_count import (CODE as DOPPELZAEHLUNG_CODE, Konflikt,
@@ -227,7 +227,7 @@ class LoanIn(BaseModel):
     name: str
     principal: Decimal
     annual_rate_pct: Decimal
-    term_months: int
+    term_months: int = Field(ge=1, le=1200)
     payout_date: date
     repayment_type: Literal["annuity", "bullet"] = "annuity"
 
@@ -245,7 +245,7 @@ class LoanPatch(BaseModel):
     name: str | None = None
     principal: Decimal | None = None
     annual_rate_pct: Decimal | None = None
-    term_months: int | None = None
+    term_months: int | None = Field(default=None, ge=1, le=1200)
     payout_date: date | None = None
     repayment_type: Literal["annuity", "bullet"] | None = None
     doppelzaehlung_bestaetigt: bool = False
@@ -318,8 +318,7 @@ def get_loan_schedule(loan_id: int, session: Session = Depends(get_session)):
     loan = session.get(Loan, loan_id)
     if loan is None:
         raise HTTPException(404, "Kredit nicht gefunden")
-    plan = loan_schedule(Decimal(loan.principal), Decimal(loan.annual_rate_pct),
-                         loan.term_months, loan.payout_date, loan.repayment_type)
+    plan = schedule_for(loan)
     return [InstallmentOut(due=i.due, payment=i.payment, interest=i.interest,
                            principal=i.principal, remaining=i.remaining)
            for i in plan]