refactor: Feed uses SocialIdentity matching, deps From impl, remove get_accepted_following_urls

Feed builds FollowingFilter by matching SocialIdentity::Local/Remote
instead of URL-prefix heuristic. Removed get_accepted_following_urls
from SocialQuery (no longer needed). Added From<&AppState> for deps
structs — 20 construction sites collapsed to one-liners.

crates/domain/src/testing/in_memory.rs

@@ -1106,10 +1106,4 @@ impl SocialQuery for InMemorySocialRepository {
         }))
     }
 
-    async fn get_accepted_following_urls(
-        &self,
-        _user_id: &UserId,
-    ) -> Result<Vec<String>, DomainError> {
-        Ok(vec![])
-    }
 }