Support cross-origin frontends alongside same-origin setups

- django-cors-headers with env-driven CORS_ALLOWED_ORIGINS,
  CORS_ALLOW_ALL_ORIGINS, CORS_ALLOW_CREDENTIALS and CSRF_TRUSTED_ORIGINS;
  same-origin traffic is unaffected and a disallowed origin gets no CORS
  headers. Token auth needs no cookies, so credentials stay off by default.
- TRUST_PROXY_HEADERS=true lets a TLS-terminating proxy supply
  X-Forwarded-Proto/Host for correct absolute URLs.
- API media URLs (raw/thumbnail/upload/similarity/staged previews) are now
  absolute, built from the request host, so <img>/<video>/fetch() keep
  working when the SPA is served from another origin. Signed URLs are still
  per-user; nothing is stored in the DB.
- The SPA gains VITE_API_BASE (build-time, empty = same-origin) applied by
  a small apiUrl() helper used for XHR/fetch and the few URL fallbacks.

Verified with a throwaway instance: preflight and GET responses carry the
allowed origin, foreign origins get nothing, media GETs include CORS for
cross-origin fetch(), and payload URLs use the request host (dev :8000
unchanged).

backend/requirements.txt

@@ -1,6 +1,7 @@
 Django>=6.0,<6.2
 djangorestframework>=3.17
 django-filter>=25.1
+django-cors-headers>=4.9
 Pillow>=11.0
 python-dotenv>=1.0
 requests>=2.32