Per-assignment times, participant avatar fix, UI improvements

- Times are now per-assignment instead of per-place, so the same place
  on different days can have different times
- Migration 26 adds assignment_time/assignment_end_time columns
- New endpoint PUT /assignments/:id/time for updating assignment times
- Time picker removed from place creation (only shown when editing)
- End-before-start validation disables save button
- Time collision warning shows overlapping activities on the same day
- Fix participant avatars using avatar_url instead of avatar filename
- Rename "Add Place" to "Add Place/Activity" (DE + EN)
- Improve README update instructions with docker inspect tip

client/src/components/Planner/DayPlanSidebar.jsx

@@ -651,7 +651,7 @@ export default function DayPlanSidebar({
                             onDragEnd={() => { setDraggingId(null); setDragOverDayId(null); setDropTargetKey(null); dragDataRef.current = null }}
                             onClick={() => { onPlaceClick(isPlaceSelected ? null : place.id, isPlaceSelected ? null : assignment.id); if (!isPlaceSelected) onSelectDay(day.id, true) }}
                             onContextMenu={e => ctxMenu.open(e, [
-                              onEditPlace && { label: t('common.edit'), icon: Pencil, onClick: () => onEditPlace(place) },
+                              onEditPlace && { label: t('common.edit'), icon: Pencil, onClick: () => onEditPlace(place, assignment.id) },
                               onRemoveAssignment && { label: t('planner.removeFromDay'), icon: Trash2, onClick: () => onRemoveAssignment(day.id, assignment.id) },
                               place.website && { label: t('inspector.website'), icon: ExternalLink, onClick: () => window.open(place.website, '_blank') },
                               (place.lat && place.lng) && { label: 'Google Maps', icon: Navigation, onClick: () => window.open(`https://www.google.com/maps/search/?api=1&query=${place.lat},${place.lng}`, '_blank') },