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/apps/library/serializers.py

@@ -103,10 +103,17 @@ class MediaItemSerializer(serializers.ModelSerializer):
         return f"J-{obj.id}"
 
     def get_raw_url(self, obj):
-        return signed_media_url(obj, self._request_user(), "raw")
+        return signed_media_url(
+            obj, self._request_user(), "raw", request=self.context.get("request")
+        )
 
     def get_thumbnail_url(self, obj):
-        return signed_media_url(obj, self._request_user(), "thumbnail")
+        return signed_media_url(
+            obj,
+            self._request_user(),
+            "thumbnail",
+            request=self.context.get("request"),
+        )
 
     def get_display_rating(self, obj):
         if obj.rating:
@@ -184,7 +191,9 @@ class TempUploadSerializer(serializers.ModelSerializer):
             {"temp": str(obj.id), "user": user.id},
             salt=UPLOAD_FILE_SALT,
         )
-        return f"/api/uploads/{obj.id}/file/?sig={signature}"
+        url = f"/api/uploads/{obj.id}/file/?sig={signature}"
+        request = self.context.get("request")
+        return request.build_absolute_uri(url) if request is not None else url
 
     def get_preview_url(self, obj):
         """A URL an <img>/<video> tag can load: staged file or library item."""
@@ -202,11 +211,9 @@ class TempUploadSerializer(serializers.ModelSerializer):
             and Path(location.rel_path).suffix.lower() in VIDEO_EXTENSIONS
         )
         action = "thumbnail" if is_video else "raw"
-        signature = signing.dumps(
-            {"item": item.id, "user": user.id, "action": action},
-            salt=MEDIA_FILE_SALT,
+        return signed_media_url(
+            item, user, action, request=self.context.get("request")
         )
-        return f"/api/files/J-{item.id}/{action}/?sig={signature}"
 
 
 class DownloadTaskSerializer(serializers.ModelSerializer):
@@ -292,7 +299,9 @@ class SimilarityCheckSerializer(serializers.ModelSerializer):
             {"check": str(obj.id), "user": user.id},
             salt=UPLOAD_FILE_SALT,
         )
-        return f"/api/similarity/{obj.id}/file/?sig={signature}"
+        url = f"/api/similarity/{obj.id}/file/?sig={signature}"
+        request = self.context.get("request")
+        return request.build_absolute_uri(url) if request is not None else url
 
     def get_expires_at(self, obj):
         ttl = int(getattr(settings, "SIMILARITY_TTL_MINUTES", 30))