feat: auto-retry uploads when rate limited (v0.13.0)

Backend: rate limiter gains check_with_retry() returning seconds until
the next slot opens. Upload 429 responses include retry_after_secs in
JSON and a Retry-After header.

Frontend: upload queue catches 429 as RateLimitError, resets affected
item to pending, schedules processQueue() for the server-reported delay,
and shows a live countdown banner in the queue UI.

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

backend/src/error.rs

@@ -1,6 +1,5 @@
 use axum::http::StatusCode;
 use axum::response::{IntoResponse, Response};
-use serde_json::json;
 
 #[derive(Debug)]
 pub enum AppError {
@@ -8,7 +7,9 @@ pub enum AppError {
     Unauthorized(String),
     Forbidden(String),
     NotFound(String),
-    TooManyRequests(String),
+    Conflict(String),
+    /// Second field: optional retry-after seconds to include in the response.
+    TooManyRequests(String, Option<u64>),
     Internal(anyhow::Error),
 }
 
@@ -19,7 +20,8 @@ impl AppError {
             Self::Unauthorized(_) => (StatusCode::UNAUTHORIZED, "unauthorized"),
             Self::Forbidden(_) => (StatusCode::FORBIDDEN, "forbidden"),
             Self::NotFound(_) => (StatusCode::NOT_FOUND, "not_found"),
-            Self::TooManyRequests(_) => (StatusCode::TOO_MANY_REQUESTS, "too_many_requests"),
+            Self::Conflict(_) => (StatusCode::CONFLICT, "conflict"),
+            Self::TooManyRequests(..) => (StatusCode::TOO_MANY_REQUESTS, "too_many_requests"),
             Self::Internal(_) => (StatusCode::INTERNAL_SERVER_ERROR, "internal_error"),
         }
     }
@@ -30,7 +32,8 @@ impl AppError {
             | Self::Unauthorized(msg)
             | Self::Forbidden(msg)
             | Self::NotFound(msg)
-            | Self::TooManyRequests(msg) => msg.clone(),
+            | Self::Conflict(msg) => msg.clone(),
+            Self::TooManyRequests(msg, _) => msg.clone(),
             Self::Internal(err) => {
                 tracing::error!("internal error: {err:#}");
                 "Ein interner Fehler ist aufgetreten.".to_string()
@@ -42,13 +45,29 @@ impl AppError {
 impl IntoResponse for AppError {
     fn into_response(self) -> Response {
         let (status, code) = self.status_and_code();
+        let retry_after_secs = if let Self::TooManyRequests(_, Some(secs)) = &self {
+            Some(*secs)
+        } else {
+            None
+        };
         let message = self.message();
-        let body = json!({
+
+        let mut body = serde_json::json!({
             "error": code,
             "message": message,
             "status": status.as_u16(),
         });
-        (status, axum::Json(body)).into_response()
+        if let Some(secs) = retry_after_secs {
+            body["retry_after_secs"] = secs.into();
+        }
+
+        let mut resp = (status, axum::Json(body)).into_response();
+        if let Some(secs) = retry_after_secs {
+            if let Ok(val) = axum::http::HeaderValue::from_str(&secs.to_string()) {
+                resp.headers_mut().insert(axum::http::header::RETRY_AFTER, val);
+            }
+        }
+        resp
     }
 }