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",
 ]