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/api/src/routers/well_known.rs

@@ -0,0 +1,71 @@
+use app::state::AppState;
+use axum::{
+    extract::{Query, State},
+    response::{IntoResponse, Json},
+};
+use serde::{Deserialize, Serialize};
+use url::Url;
+
+#[derive(Deserialize)]
+pub struct WebFingerQuery {
+    resource: String,
+}
+
+#[derive(Serialize)]
+pub struct WebFingerLink {
+    rel: String,
+    #[serde(rename = "type")]
+    type_: String,
+    href: Url,
+}
+
+#[derive(Serialize)]
+pub struct WebFingerResponse {
+    subject: String,
+    links: Vec<WebFingerLink>,
+}
+
+pub async fn webfinger(
+    State(state): State<AppState>,
+    Query(query): Query<WebFingerQuery>,
+) -> Result<impl IntoResponse, impl IntoResponse> {
+    if let Some((scheme, account_info)) = query.resource.split_once(':') {
+        if scheme != "acct" {
+            return Err((
+                axum::http::StatusCode::BAD_REQUEST,
+                "Invalid resource scheme",
+            ));
+        }
+
+        let account_parts: Vec<&str> = account_info.split('@').collect();
+        let username = account_parts[0];
+
+        let user = match app::persistence::user::get_user_by_username(&state.conn, username).await {
+            Ok(Some(user)) => user,
+            _ => return Err((axum::http::StatusCode::NOT_FOUND, "User not found")),
+        };
+
+        let base_url = "http://localhost:3000";
+        let user_url = Url::parse(&format!("{}/users/{}", base_url, user.username)).unwrap();
+
+        let response = WebFingerResponse {
+            subject: query.resource,
+            links: vec![WebFingerLink {
+                rel: "self".to_string(),
+                type_: "application/activity+json".to_string(),
+                href: user_url,
+            }],
+        };
+
+        Ok(Json(response))
+    } else {
+        Err((
+            axum::http::StatusCode::BAD_REQUEST,
+            "Invalid resource format",
+        ))
+    }
+}
+
+pub fn create_well_known_router() -> axum::Router<AppState> {
+    axum::Router::new().route("/webfinger", axum::routing::get(webfinger))
+}