Support serving iscc-web under a URL path prefix #67

Problem

iscc-web only works at the root of a domain or subdomain (https://iscc.example.com/). It cannot be served under a path prefix such as https://intranet.example.com/iscc/, which is a common requirement for internal and permissioned deployments that share one hostname.

Absolute root paths are hard-coded in several places:

• Frontend: frontend/services/api.service.ts calls /api/v1/...; Vite builds with base: "/static/dist/".
• Backend: iscc_web/vite.py (VITE_STATIC_URL = "/static/dist/"); iscc_web/api/common.py base_url() and the Location headers in iscc_web/api/iscc.py and iscc_web/api/media.py build /api/v1/... URLs; routes are mounted at /static, /docs, /images and /health in iscc_web/main.py.
• API docs page: iscc_web/static/docs/index.html loads /static/... and /docs/openapi.yaml; openapi.yaml declares servers: /api/v1 (and / for /health).

A reverse proxy that strips the prefix gets the API working, but the page and the URLs in API responses still point at the root.

Proposal

Add an option ISCC_WEB_ROOT_PATH (empty by default) for deployments behind a proxy that strips the prefix:

• Backend: prefix generated URLs (base_url(), Location headers, Vite asset tags, docs page asset paths) with the root path. ASGI root_path may cover part of this.
• Frontend: read the prefix from the injected window.__ISCC_WEB__ config instead of hard-coding /api/v1, and build assets relative to it.
• Docs page: serve it through the template, or make its asset and spec paths relative.

Done when

• The demo page, uploads, downloads, metadata embedding, /docs and /health work under a prefix behind a stripping proxy, and the URLs in API responses carry the prefix.
• The default (no prefix) behaves exactly as today.
• Tests cover URL generation with and without a root path; the deployment docs show a prefixed proxy configuration.

Until then, the "Run behind a login" guide documents that iscc-web needs its own domain or subdomain.