Reservation end time, route perf overhaul, assignment search fix

- Add reservation_end_time field (DB migration, API, UI)
- Split reservation form: separate date, start time, end time, status fields
- Fix DateTimePicker forcing 00:00 when no time selected
- Show end time across all reservation displays
- Link-to-assignment and date on same row (50/50 layout)
- Assignment search now shows day headers for filtered results
- Auto-fill date when selecting a day assignment
- Route segments: single OSRM request instead of N separate calls (~6s → ~1s)
- Route labels visible from zoom level 12 (was 16)
- Fix stale route labels after place deletion (useEffect triggers recalc)
- AbortController cancels outdated route calculations

client/src/i18n/translations/de.js

@@ -577,6 +577,8 @@ const de = {
   'reservations.editTitle': 'Reservierung bearbeiten',
   'reservations.status': 'Status',
   'reservations.datetime': 'Datum & Uhrzeit',
+  'reservations.startTime': 'Startzeit',
+  'reservations.endTime': 'Endzeit',
   'reservations.date': 'Datum',
   'reservations.time': 'Uhrzeit',
   'reservations.timeAlt': 'Uhrzeit (alternativ, z.B. 19:30)',

client/src/i18n/translations/en.js

@@ -577,6 +577,8 @@ const en = {
   'reservations.editTitle': 'Edit Reservation',
   'reservations.status': 'Status',
   'reservations.datetime': 'Date & Time',
+  'reservations.startTime': 'Start time',
+  'reservations.endTime': 'End time',
   'reservations.date': 'Date',
   'reservations.time': 'Time',
   'reservations.timeAlt': 'Time (alternative, e.g. 19:30)',