Refactor gateway into modular architecture

Split 1878-line server.py into 15 focused modules:
- config.py: all env vars and constants
- database.py: schema, init, seed logic
- sessions.py: session/token CRUD
- proxy.py: proxy_request, SERVICE_MAP, resolve_service
- responses.py: ResponseMixin for handler helpers
- auth.py: login/logout/register handlers
- dashboard.py: dashboard, apps, connections, pinning
- command.py: AI command bar
- integrations/booklore.py: auth, books, cover, import
- integrations/kindle.py: send-to-kindle, file finder
- integrations/karakeep.py: save/delete bookmarks
- integrations/qbittorrent.py: download status
- integrations/image_proxy.py: external image proxy

server.py is now thin routing only (~344 lines).
All routes, methods, status codes, and responses preserved exactly.
Added PYTHONUNBUFFERED=1 to Dockerfile for live logging.

gateway/Dockerfile

@@ -1,5 +1,7 @@
 FROM python:3.12-slim
 WORKDIR /app
-COPY server.py .
+COPY server.py config.py database.py sessions.py proxy.py responses.py auth.py dashboard.py command.py ./
+COPY integrations/ ./integrations/
 EXPOSE 8100
+ENV PYTHONUNBUFFERED=1
 CMD ["python3", "server.py"]