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>

frontend/src/types/specialAssignment.ts

@@ -0,0 +1,51 @@
+export type AssignmentMode = 'fza' | 'payroll' | 'both';
+
+export interface SpecialAssignmentOut {
+  id: string;
+  user_id: string;
+  company_id: string;
+  date_from: string;
+  date_to: string;
+  factor: number;
+  mode: AssignmentMode;
+  description: string | null;
+  label: string | null;
+}
+
+export interface SpecialAssignmentCreate {
+  date_from: string;
+  date_to: string;
+  factor: number;
+  mode: AssignmentMode;
+  description?: string;
+  label?: string;
+}
+
+// ── Payroll Report ────────────────────────────────────────────────────────────
+
+export interface PayrollAssignmentEntry {
+  assignment_id: string;
+  label: string | null;
+  date_from: string;
+  date_to: string;
+  factor: number;
+  normal_hours: number;
+  factor_hours: number;
+  extra_hours: number;
+}
+
+export interface PayrollAssignmentRow {
+  user_id: string;
+  user_name: string;
+  personnel_number: string | null;
+  assignments: PayrollAssignmentEntry[];
+  total_normal_hours: number;
+  total_factor_hours: number;
+  total_extra_hours: number;
+}
+
+export interface PayrollAssignmentReport {
+  year: number;
+  month: number;
+  rows: PayrollAssignmentRow[];
+}