feat: Sondervertretungs-Faktoren (special_assignments)

- Neues Model SpecialAssignment mit AssignmentMode (fza|payroll|both)
- CRUD-Endpunkte unter /users/{id}/special-assignments
- Payroll-Report: GET /reports/special-assignments/payroll?year=&month=
- Migration 0029: special_assignments Tabelle + btree_gist Overlap-Constraint
- _recalculate_overtime_balance berücksichtigt FZA-Faktoren
- Frontend: Sondervertretungs-Zeiträume im UsersPage Edit-Modal
- Frontend: ReportsPage neuer Tab 'Sondervertretungen' mit Payroll-Tabelle + CSV-Export

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -14,6 +14,7 @@ from app.models.public_holiday import PublicHoliday
 from app.models.smtp_config import SmtpConfig
 from app.models.caldav_config import CaldavCompanyConfig, CaldavUserConfig
 from app.models.kiosk_device import KioskDevice, KioskAuthMethod
+from app.models.special_assignment import SpecialAssignment, AssignmentMode
 
 __all__ = [
     "Company",
@@ -35,4 +36,6 @@ __all__ = [
     "PublicHoliday",
     "KioskDevice",
     "KioskAuthMethod",
+    "SpecialAssignment",
+    "AssignmentMode",
 ]

backend/app/models/special_assignment.py

@@ -0,0 +1,38 @@
+"""Sondervertretungs-Faktoren: Per-User Zeitraum-Zuweisung mit Multiplikator."""
+import enum
+from datetime import date
+from decimal import Decimal
+from uuid import UUID, uuid4
+
+from sqlalchemy import CheckConstraint, Date, Enum, ForeignKey, Numeric, String, Text
+from sqlalchemy.dialects.postgresql import UUID as PG_UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.database import Base
+
+
+class AssignmentMode(str, enum.Enum):
+    fza = "fza"          # Nur FZA-Stunden-Anrechnung
+    payroll = "payroll"  # Nur Gehaltsabrechnung (Bericht)
+    both = "both"        # Beides
+
+
+class SpecialAssignment(Base):
+    """Sondervertretungs-Zeitraum mit Faktor für einen Mitarbeiter."""
+    __tablename__ = "special_assignments"
+    __table_args__ = (
+        CheckConstraint("factor > 0 AND factor <= 10", name="ck_special_assignment_factor"),
+        CheckConstraint("date_from <= date_to", name="ck_special_assignment_dates"),
+    )
+
+    id: Mapped[UUID] = mapped_column(PG_UUID(as_uuid=True), primary_key=True, default=uuid4)
+    user_id: Mapped[UUID] = mapped_column(PG_UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    company_id: Mapped[UUID] = mapped_column(PG_UUID(as_uuid=True), ForeignKey("companies.id", ondelete="CASCADE"), nullable=False, index=True)
+
+    date_from: Mapped[date] = mapped_column(Date, nullable=False)
+    date_to: Mapped[date] = mapped_column(Date, nullable=False)
+
+    factor: Mapped[Decimal] = mapped_column(Numeric(5, 3), nullable=False)
+    mode: Mapped[AssignmentMode] = mapped_column(Enum(AssignmentMode, name="assignment_mode"), nullable=False, default=AssignmentMode.both)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    label: Mapped[str | None] = mapped_column(String(100), nullable=True)  # z.B. "Schichtleiter Vertretung"