receipts: Set Cache-Control header on photos

Some of the receipt photos can be quite large.  Since there's no
(direct) way to change receipt photos, we can tell the browser to cache
them for a long time to avoid having to download them every time the
list page is shown.

src/routes/receipts.rs

@@ -1,5 +1,5 @@
 use rocket::form::Form;
-use rocket::http::{ContentType, MediaType, Status};
+use rocket::http::{ContentType, Header, MediaType, Status};
 use rocket::serde::json::Json;
 use rocket::{Route, State};
 use rocket_db_pools::Connection as DatabaseConnection;
@@ -118,11 +118,25 @@ pub async fn get_receipt(
     }
 }
 
+#[derive(rocket::response::Responder)]
+pub struct PhotoResponse {
+    content: Vec<u8>,
+    content_type: ContentType,
+    cache_control: Header<'static>,
+}
+
+impl PhotoResponse {
+    fn new(content: Vec<u8>, content_type: ContentType) -> Self {
+        let cache_control = Header::new("Cache-Control", "max-age=604800");
+        Self { content, content_type, cache_control }
+    }
+}
+
 #[rocket::get("/<id>/view/<_>")]
 pub async fn view_receipt_photo(
     id: i32,
     db: DatabaseConnection<Database>,
-) -> Option<(ContentType, Vec<u8>)> {
+) -> Option<PhotoResponse> {
     let mut repo = ReceiptsRepository::new(db);
     match repo.get_receipt_photo(id).await {
         Ok((filename, image)) => {
@@ -130,7 +144,7 @@ pub async fn view_receipt_photo(
                 .rsplit_once('.')
                 .and_then(|(_, ext)| MediaType::from_extension(ext))
                 .unwrap_or(MediaType::Binary);
-            Some((ContentType(mt), image))
+            Some(PhotoResponse::new(image, ContentType(mt)))
         },
         Err(e) => {
             error!("Error fetching receipt image: {}", e);