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/main.py

@@ -15,6 +15,7 @@ from app.routers import import_kimai
 from app.routers import kiosk
 from app.routers import busylight
 from app.routers import audit
+from app.routers import special_assignments
 
 
 @asynccontextmanager
@@ -77,6 +78,7 @@ app.include_router(import_kimai.router,  prefix=API_PREFIX)
 app.include_router(kiosk.router,         prefix=API_PREFIX)
 app.include_router(busylight.router,     prefix=API_PREFIX)
 app.include_router(audit.router,         prefix=API_PREFIX)
+app.include_router(special_assignments.router, prefix=API_PREFIX)
 
 
 # ── Health ────────────────────────────────────────────────────────────────────