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/i18n/translations/de.js

@@ -504,7 +504,7 @@ const de = {
   'dayplan.pdfError': 'Fehler beim PDF-Export',
 
   // Places Sidebar
-  'places.addPlace': 'Ort hinzufügen',
+  'places.addPlace': 'Ort/Aktivität hinzufügen',
   'places.assignToDay': 'Zu welchem Tag hinzufügen?',
   'places.all': 'Alle',
   'places.unplanned': 'Ungeplant',
@@ -529,6 +529,8 @@ const de = {
   'places.formTime': 'Uhrzeit',
   'places.startTime': 'Start',
   'places.endTime': 'Ende',
+  'places.endTimeBeforeStart': 'Endzeit liegt vor der Startzeit',
+  'places.timeCollision': 'Zeitliche Überschneidung mit:',
   'places.formWebsite': 'Website',
   'places.formNotesPlaceholder': 'Persönliche Notizen...',
   'places.formReservation': 'Reservierung',
@@ -864,8 +866,8 @@ const de = {
   'planner.placeN': '{n} Orte',
   'planner.addNote': 'Notiz hinzufügen',
   'planner.noEntries': 'Keine Einträge für diesen Tag',
-  'planner.addPlace': 'Ort hinzufügen',
-  'planner.addPlaceShort': '+ Ort hinzufügen',
+  'planner.addPlace': 'Ort/Aktivität hinzufügen',
+  'planner.addPlaceShort': '+ Ort/Aktivität hinzufügen',
   'planner.resPending': 'Reservierung ausstehend · ',
   'planner.resConfirmed': 'Reservierung bestätigt · ',
   'planner.notePlaceholder': 'Notiz\u2026',

client/src/i18n/translations/en.js

@@ -504,7 +504,7 @@ const en = {
   'dayplan.pdfError': 'Failed to export PDF',
 
   // Places Sidebar
-  'places.addPlace': 'Add Place',
+  'places.addPlace': 'Add Place/Activity',
   'places.assignToDay': 'Add to which day?',
   'places.all': 'All',
   'places.unplanned': 'Unplanned',
@@ -529,6 +529,8 @@ const en = {
   'places.formTime': 'Time',
   'places.startTime': 'Start',
   'places.endTime': 'End',
+  'places.endTimeBeforeStart': 'End time is before start time',
+  'places.timeCollision': 'Time overlap with:',
   'places.formWebsite': 'Website',
   'places.formNotesPlaceholder': 'Personal notes...',
   'places.formReservation': 'Reservation',
@@ -864,8 +866,8 @@ const en = {
   'planner.placeN': '{n} places',
   'planner.addNote': 'Add note',
   'planner.noEntries': 'No entries for this day',
-  'planner.addPlace': 'Add place',
-  'planner.addPlaceShort': '+ Add place',
+  'planner.addPlace': 'Add place/activity',
+  'planner.addPlaceShort': '+ Add place/activity',
   'planner.resPending': 'Reservation pending · ',
   'planner.resConfirmed': 'Reservation confirmed · ',
   'planner.notePlaceholder': 'Note\u2026',