Replace internal todos with Vikunja API wrapper

- New app/vikunja.py: get_tasks, create_task, update_task, delete_task
- /todos page now reads from and writes to Vikunja (create, mark done, delete)
- Auto-todos (clean build plate, filament low) post to Vikunja after DB commit
- /api/todos router proxies to Vikunja
- .env.install: VIKUNJA_URL, VIKUNJA_API_TOKEN, VIKUNJA_PROJECT_ID

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

.env.install

@@ -6,6 +6,9 @@ MYSQL_PASSWORD=
 STL_UPLOAD_DIR=/data/stl
 TZ=Europe/Berlin
 FILAMENTDB_URL=https://web.filamentdb.orb.local
+VIKUNJA_URL=https://vikunja.example.com
+VIKUNJA_API_TOKEN=
+VIKUNJA_PROJECT_ID=
 VAT_RATE=19
 FILAMENT_PRICE_PER_KG=15
 MATERIAL_FACTOR=0.50