feat: implement merge readiness plan to close gaps between v2 and v1

- Task 1: Fix feed response hydration by adding `to_thought_response` helper and updating feed handlers to return full `ThoughtResponse`.
- Task 2: Wire follower/following REST routes for user feeds.
- Task 3: Add user listing and count endpoints, including `GET /users` and `GET /users/count`.
- Task 4: Implement popular tags feature with `GET /tags/popular`.
- Task 5: Enhance configuration with HOST, CORS_ORIGINS, and optional rate limiting using tower-governor.

crates/adapters/activitypub/src/urls.rs

@@ -6,7 +6,9 @@ pub struct ThoughtsUrls {
 
 impl ThoughtsUrls {
     pub fn new(base_url: &str) -> Self {
-        Self { base_url: base_url.trim_end_matches('/').to_string() }
+        Self {
+            base_url: base_url.trim_end_matches('/').to_string(),
+        }
     }
 
     pub fn user_url(&self, username: &str) -> Url {
@@ -37,13 +39,19 @@ mod tests {
     #[test]
     fn user_url_format() {
         let urls = ThoughtsUrls::new("https://example.com");
-        assert_eq!(urls.user_url("alice").as_str(), "https://example.com/users/alice");
+        assert_eq!(
+            urls.user_url("alice").as_str(),
+            "https://example.com/users/alice"
+        );
     }
 
     #[test]
     fn thought_url_format() {
         let urls = ThoughtsUrls::new("https://example.com");
         let id = uuid::Uuid::nil();
-        assert!(urls.thought_url(id).as_str().starts_with("https://example.com/thoughts/"));
+        assert!(urls
+            .thought_url(id)
+            .as_str()
+            .starts_with("https://example.com/thoughts/"));
     }
 }