feat(feed): operator-configurable public feed advertised via /capabilities

A read-only web feed (feed.<domain>) is a backend-operator knob: FEED_ENABLED
plus curation switches (show owner / show premium members / a featured npub
allowlist / extra relays) surface in /capabilities.feed so the feed page adapts
per instance. Reads from the relay, so validate() fail-closes FEED_ENABLED
without RELAY_ENABLED. Off by default; all public display config, no secrets.

src/api/openapi.rs

@@ -77,6 +77,7 @@ use utoipa::OpenApi;
         crate::api::capabilities::MessagingCapability,
         crate::api::capabilities::PremiumCapability,
         crate::api::capabilities::PremiumPlanInfo,
+        crate::api::capabilities::FeedCapability,
         crate::api::prices::PricesResponse,
         crate::api::auth::AuthResponse,
         crate::api::auth::UserInfo,