fix: Resolve 75 compile errors across 13 backend files

Remove deprecated #[axum::async_trait] from 2 middleware files (native
async traits on Rust 1.88+). Fix 71 stub handlers: change return type
from ApiResult<impl IntoResponse> to ApiResult<Json<Value>> and replace
todo!() with proper JSON stub responses. Clean compile, zero errors.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/middleware/auth.rs

@@ -17,7 +17,6 @@ pub struct AuthUser {
     pub role: Option<String>,
 }
 
-#[axum::async_trait]
 impl FromRequestParts<Arc<AppState>> for AuthUser {
     type Rejection = http::StatusCode;
 

backend/src/middleware/license.rs

@@ -19,7 +19,6 @@ pub struct ValidLicense {
 // - Populate the modules list for downstream permission checks
 // - Return 403 Forbidden if the license is invalid, expired, or suspended
 
-#[axum::async_trait]
 impl<S> FromRequestParts<S> for ValidLicense
 where
     S: Send + Sync,