refactor: extract handler business logic into application use cases

11 handlers were calling repos/ports directly, bypassing the
application layer. Extracted into proper use cases:

feed: get_public_feed, get_user_feed, get_tag_feed, get_popular_tags
profile: get_user_profile (with follow check), list_users,
         count_local_users, list_local_followers, list_local_following
federation_management: set_also_known_as

Also registers 9 previously undocumented handlers in OpenAPI modules.

crates/application/src/use_cases/feed.rs

@@ -1,7 +1,7 @@
 use domain::{
     errors::DomainError,
     models::feed::{FeedEntry, PageParams, Paginated},
-    ports::{FeedOptions, FeedQuery, FeedRepository, FeedRequest, FollowRepository},
+    ports::{FeedOptions, FeedQuery, FeedRepository, FeedRequest, FollowRepository, TagRepository},
     value_objects::UserId,
 };
 
@@ -20,3 +20,51 @@ pub async fn get_home_feed(
     })
     .await
 }
+
+pub async fn get_public_feed(
+    feed: &dyn FeedRepository,
+    viewer: Option<UserId>,
+    page: PageParams,
+    opts: FeedOptions,
+) -> Result<Paginated<FeedEntry>, DomainError> {
+    feed.query(&FeedRequest {
+        query: FeedQuery::public(page, viewer),
+        options: opts,
+    })
+    .await
+}
+
+pub async fn get_user_feed(
+    feed: &dyn FeedRepository,
+    user_id: UserId,
+    page: PageParams,
+    opts: FeedOptions,
+    viewer: Option<UserId>,
+) -> Result<Paginated<FeedEntry>, DomainError> {
+    feed.query(&FeedRequest {
+        query: FeedQuery::user(user_id, page, viewer),
+        options: opts,
+    })
+    .await
+}
+
+pub async fn get_tag_feed(
+    feed: &dyn FeedRepository,
+    tag: &str,
+    page: PageParams,
+    opts: FeedOptions,
+    viewer: Option<UserId>,
+) -> Result<Paginated<FeedEntry>, DomainError> {
+    feed.query(&FeedRequest {
+        query: FeedQuery::tag(tag, page, viewer),
+        options: opts,
+    })
+    .await
+}
+
+pub async fn get_popular_tags(
+    tags: &dyn TagRepository,
+    limit: usize,
+) -> Result<Vec<(String, i64)>, DomainError> {
+    tags.popular_tags(limit).await
+}