docs: add troubleshooting for DB password auth failure on stale volume

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -54,3 +54,28 @@ All secrets via environment variables or `.env` (never committed):
 | `JELLYFIN_URL` | Jellyfin base URL |
 | `JELLYFIN_API_KEY` | Jellyfin API key |
 | `MEDIA_PATH` | Local path for downloaded media |
+
+## Troubleshooting
+
+### `password authentication failed for user "kino"`
+
+The PostgreSQL data volume was initialized with a different password than what
+the backend is currently configured to use. This happens when the `.env` file
+is created, changed, or removed after the first `docker compose up`.
+
+Reset the volume and restart:
+
+```bash
+docker compose down -v   # removes all volumes including db_data — data will be lost
+docker compose up --build
+```
+
+If you want to keep data, first export it:
+
+```bash
+docker compose exec db pg_dump -U kino kino > backup.sql
+docker compose down -v
+docker compose up --build
+# restore:
+docker compose exec -T db psql -U kino kino < backup.sql
+```