chore(rebrand): rename WatchStack -> Quivio

Quivio (Kofferwort aus Quire + Video) ersetzt den Arbeitstitel WatchStack.
Es ist origineller, frei auf PyPI/npm, und passt perfekt zur Doppelnatur
der App: Buecher (Quire) + Serien (Video).

Geaendert:
- Python-Paket app/ -> quivio/ (+ alle Imports angepasst)
- DB-File watchstack.db -> quivio.db
- Docker-Image: quivio:0.5.1-beta, Container-Name quivio
- Compose-Services: quivio, quivio-local
- ENV-Vars: QUIVIO_VERSION, QUIVIO_PORT, Volume quivio-data
- Frontend-Logo "W" -> "Q", Titel "Quivio", Footer
- Favicon "Q" (Georgia serif fuer klassischen Look)
- Logger-Name: watchstack -> quivio
- README, NOTICE, dist/README, data/README: Quivio
- CI-Workflow: testet quivio/ Pfade
- build-and-push.sh: lokaler Tag "quivio:VERSION"
- service-Feld in /api/health: quivio
- OpenAPI title: Quivio

Bugfix (gefunden beim Renaming):
- seed.py hatte versteckten "from app.database import DB_PATH" — gefixt

Verifiziert end-to-end:
- Python-Import OK
- Backend: service=quivio, version=0.5.1-beta, 8 unique items
- Docker-Build OK, Push in Gitea-Registry OK (neuer sha256-Digest)
- Container: Quivio 0.5.1-beta ready (logger), HTML ohne WatchStack-Rest
- OpenAPI title=Quivio
- 8 unique items nach cold start (Race-Fix haelt)

.github/workflows/ci.yml

@@ -18,12 +18,12 @@ jobs:
       - run: pip install -r requirements.txt
       - name: Backend-Import-Check
         run: |
-          python -c "from app.main import app; print('OK', app.title, app.version)"
+          python -c "from quivio.main import app; print('OK', app.title, app.version)"
       - name: Frontend-Asset-Check
         run: |
-          test -f app/static/index.html
-          test -f app/static/css/style.css
-          test -f app/static/js/app.js
+          test -f quivio/static/index.html
+          test -f quivio/static/css/style.css
+          test -f quivio/static/js/app.js
 
   # Hinweis: Push in die Gitea Container Registry laeuft lokal ueber
   #   bin/build-and-push.sh