Route trending feed requests through Bluesky appview
Add getProxyHeadersForFeed() helper that detects trending feed URIs
and sets atproto-proxy to Bluesky's appview. The Blacksky appview
cannot resolve did:web:topics.bsky.app (Bluesky's trending feed
service), causing 400 errors for trending topic feeds.
Applied to CustomFeedAPI, MergeFeedAPI, and useFeedSourceInfoQuery.