refactor: unify authentication state and improve middleware efficiency

- Introduce AuthInfo struct for shared auth state.
- Populate AuthInfo in rate_limit_middleware and store in request extensions.
- Update AuthenticatedClient extractor to use pre-resolved AuthInfo.
- Simplify chat_completions by removing redundant DB lookups.

src/auth/mod.rs

@@ -1,33 +1,40 @@
 use axum::{extract::FromRequestParts, http::request::Parts};
-use axum_extra::TypedHeader;
-use axum_extra::headers::Authorization;
-use headers::authorization::Bearer;
 
 use crate::errors::AppError;
 
-pub struct AuthenticatedClient {
+#[derive(Debug, Clone)]
+pub struct AuthInfo {
     pub token: String,
     pub client_id: String,
 }
 
+pub struct AuthenticatedClient {
+    pub info: AuthInfo,
+}
+
 impl<S> FromRequestParts<S> for AuthenticatedClient
 where
     S: Send + Sync,
 {
     type Rejection = AppError;
 
-    async fn from_request_parts(parts: &mut Parts, state: &S) -> Result<Self, Self::Rejection> {
-        // Extract bearer token from Authorization header
-        let TypedHeader(Authorization(bearer)) = TypedHeader::<Authorization<Bearer>>::from_request_parts(parts, state)
-            .await
-            .map_err(|_| AppError::AuthError("Missing or invalid bearer token".to_string()))?;
+    async fn from_request_parts(parts: &mut Parts, _state: &S) -> Result<Self, Self::Rejection> {
+        // Retrieve AuthInfo from request extensions, where it was placed by rate_limit_middleware
+        let info = parts
+            .extensions
+            .get::<AuthInfo>()
+            .cloned()
+            .ok_or_else(|| AppError::AuthError("Authentication info not found in request".to_string()))?;
 
-        let token = bearer.token().to_string();
+        Ok(AuthenticatedClient { info })
+    }
+}
 
-        // Derive client_id from the token prefix
-        let client_id = format!("client_{}", &token[..8.min(token.len())]);
+impl std::ops::Deref for AuthenticatedClient {
+    type Target = AuthInfo;
 
-        Ok(AuthenticatedClient { token, client_id })
+    fn deref(&self) -> &Self::Target {
+        &self.info
     }
 }