feat: Implement WebFinger discovery and ActivityPub user actor endpoint

- Added a new router for handling well-known endpoints, specifically WebFinger.
- Implemented the `webfinger` function to respond to WebFinger queries.
- Created a new route for WebFinger in the router.
- Refactored user retrieval logic to support both user ID and username in a single endpoint.
- Updated user router to use the new `get_user_by_param` function.
- Added tests for WebFinger discovery and ActivityPub user actor endpoint.
- Updated dependencies in Cargo.toml files to include necessary libraries.

thoughts-backend/tests/api/activitypub.rs

@@ -0,0 +1,59 @@
+use crate::api::main::{create_user_with_password, setup};
+use axum::http::{header, StatusCode};
+use http_body_util::BodyExt;
+use serde_json::Value;
+use utils::testing::{make_get_request, make_request_with_headers};
+
+#[tokio::test]
+async fn test_webfinger_discovery() {
+    let app = setup().await;
+    create_user_with_password(&app.db, "testuser", "password123").await;
+
+    // 1. Valid WebFinger lookup for existing user
+    let url = "/.well-known/webfinger?resource=acct:testuser@localhost:3000";
+    let response = make_get_request(app.router.clone(), url, None).await;
+    assert_eq!(response.status(), StatusCode::OK);
+    let body = response.into_body().collect().await.unwrap().to_bytes();
+    let v: Value = serde_json::from_slice(&body).unwrap();
+    assert_eq!(v["subject"], "acct:testuser@localhost:3000");
+    assert_eq!(
+        v["links"][0]["href"],
+        "http://localhost:3000/users/testuser"
+    );
+
+    // 2. WebFinger lookup for a non-existent user
+    let response = make_get_request(
+        app.router.clone(),
+        "/.well-known/webfinger?resource=acct:nobody@localhost:3000",
+        None,
+    )
+    .await;
+    assert_eq!(response.status(), StatusCode::NOT_FOUND);
+}
+
+#[tokio::test]
+async fn test_user_actor_endpoint() {
+    let app = setup().await;
+    create_user_with_password(&app.db, "testuser", "password123").await;
+
+    let response = make_request_with_headers(
+        app.router.clone(),
+        "/users/testuser",
+        "GET",
+        None,
+        vec![(
+            header::ACCEPT,
+            "application/activity+json, application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"",
+        )],
+    ).await;
+
+    assert_eq!(response.status(), StatusCode::OK);
+    let content_type = response.headers().get(header::CONTENT_TYPE).unwrap();
+    assert_eq!(content_type, "application/activity+json");
+
+    let body = response.into_body().collect().await.unwrap().to_bytes();
+    let v: Value = serde_json::from_slice(&body).unwrap();
+    assert_eq!(v["type"], "Person");
+    assert_eq!(v["preferredUsername"], "testuser");
+    assert_eq!(v["id"], "http://localhost:3000/users/testuser");
+}