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/PlaceInspector.jsx

@@ -550,7 +550,7 @@ function ParticipantsBox({ tripMembers, participantIds, allJoined, onSetParticip
                 display: 'flex', alignItems: 'center', justifyContent: 'center', fontSize: 7, fontWeight: 700,
                 color: 'var(--text-muted)', overflow: 'hidden', flexShrink: 0,
               }}>
-                {member.avatar ? <img src={member.avatar} style={{ width: '100%', height: '100%', objectFit: 'cover' }} /> : member.username?.[0]?.toUpperCase()}
+                {(member.avatar_url || member.avatar) ? <img src={member.avatar_url || `/uploads/avatars/${member.avatar}`} style={{ width: '100%', height: '100%', objectFit: 'cover' }} /> : member.username?.[0]?.toUpperCase()}
               </div>
               <span style={{ textDecoration: isHovered && canRemove ? 'line-through' : 'none' }}>{member.username}</span>
             </div>
@@ -590,7 +590,7 @@ function ParticipantsBox({ tripMembers, participantIds, allJoined, onSetParticip
                       display: 'flex', alignItems: 'center', justifyContent: 'center', fontSize: 8, fontWeight: 700,
                       color: 'var(--text-muted)', overflow: 'hidden', flexShrink: 0,
                     }}>
-                      {member.avatar ? <img src={member.avatar} style={{ width: '100%', height: '100%', objectFit: 'cover' }} /> : member.username?.[0]?.toUpperCase()}
+                      {(member.avatar_url || member.avatar) ? <img src={member.avatar_url || `/uploads/avatars/${member.avatar}`} style={{ width: '100%', height: '100%', objectFit: 'cover' }} /> : member.username?.[0]?.toUpperCase()}
                     </div>
                     {member.username}
                   </button>