Tag all Vikunja tasks with #POPS label; fix Vikunja URL

- On create_task(), look up the 'POPS' label (create it if missing) and
  attach it to every task. Label ID is cached for the process lifetime.
- Corrected VIKUNJA_URL typo (tasks → task)

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

.env.install

@@ -6,7 +6,7 @@ MYSQL_PASSWORD=
 STL_UPLOAD_DIR=/data/stl
 TZ=Europe/Berlin
 FILAMENTDB_URL=https://web.filamentdb.orb.local
-VIKUNJA_URL=https://vikunja.example.com
+VIKUNJA_URL=https://task.example.com
 VIKUNJA_API_TOKEN=
 VIKUNJA_PROJECT_ID=
 VAT_RATE=19