feat(config): add rate limit configuration to AppConfig and update related usages

crates/application/src/config.rs

@@ -2,6 +2,7 @@
 pub struct AppConfig {
     pub allow_registration: bool,
     pub base_url: String,
+    pub rate_limit: u64,
 }
 
 impl AppConfig {
@@ -11,6 +12,10 @@ impl AppConfig {
             .unwrap_or(false);
         let base_url = std::env::var("BASE_URL")
             .unwrap_or_else(|_| "http://localhost:3000".to_string());
-        Self { allow_registration, base_url }
+        let rate_limit = std::env::var("RATE_LIMIT")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(20);
+        Self { allow_registration, base_url, rate_limit }
     }
 }