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

README.md

@@ -131,15 +131,23 @@ docker compose up -d
 
 ### Updating
 
+**Docker Compose** (recommended):
+
+```bash
+docker compose pull && docker compose up -d
+```
+
+**Docker Run** — use the same volume paths from your original `docker run` command:
+
 ```bash
 docker pull mauriceboe/nomad
 docker rm -f nomad
-docker run -d --name nomad -p 3000:3000 -v /your/data:/app/data -v /your/uploads:/app/uploads --restart unless-stopped mauriceboe/nomad
+docker run -d --name nomad -p 3000:3000 -v ./data:/app/data -v ./uploads:/app/uploads --restart unless-stopped mauriceboe/nomad
 ```
 
-Or with Docker Compose: `docker compose pull && docker compose up -d`
+> **Tip:** Not sure which paths you used? Run `docker inspect nomad --format '{{json .Mounts}}'` before removing the container.
 
-Your data is persisted in the mounted `data` and `uploads` volumes.
+Your data is persisted in the mounted `data` and `uploads` volumes — updates never touch your existing data.
 
 ### Reverse Proxy (recommended)