Stop throttling signed media and ease the browser's e621 queue

Signed media URLs are fetched by <img>/<video> tags without an
Authorization header, so they were charged to the anonymous 120/min
bucket: past that, galleries and the fish-greeting download got 429 JSON
instead of image bytes. The raw/thumbnail/staged-file/similarity-file
actions are now exempt, and THROTTLE_ENABLED=false removes the general
anon+user limits for private/tailnet deployments (login/register/proxy
guards stay).

The SPA's e621 client also stops self-throttling so hard: 1s gap between
browsing calls (2.5s for the stricter IQDB endpoint) and a 15s cooldown
instead of 60s when e621 answers 429.

backend/apps/library/similarity.py

@@ -136,7 +136,12 @@ class SimilarityCheckViewSet(
             self.get_serializer(check).data, status=status.HTTP_201_CREATED
         )
 
-    @action(detail=True, methods=["get", "head"], permission_classes=[AllowAny])
+    @action(
+        detail=True,
+        methods=["get", "head"],
+        permission_classes=[AllowAny],
+        throttle_classes=[],
+    )
     def file(self, request, pk=None):
         """Serve the temp file; accepts a signed URL like staged uploads."""
         check = None